What we do now:
We use a static record size of 4K. This gives a good balance of latency and
throughput.

Optimize latency:
By initialy sending small (1 TCP segment) sized records, we are able to avoid
HoL blocking of the first byte. This means TTFB is sometime lower by a whole
RTT.

Optimizing throughput:
By sending increasingly larger records later in the connection, when HoL is not
a problem, we reduce the overhead of TLS record (29 bytes per record with
GCM/CHACHA-POLY).

Logic:
Start each connection with small records (1369 byte default, change with
ssl_dyn_rec_size_lo). After a given number of records (40, change with
ssl_dyn_rec_threshold) start sending larger records (4229, ssl_dyn_rec_size_hi).
Eventually after the same number of records, start sending the largest records
(ssl_buffer_size).
In case the connection idles for a given amount of time (1s,
ssl_dyn_rec_timeout), the process repeats itself (i.e. begin sending small
records again).


diff --color -uNr a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
--- a/src/event/ngx_event_openssl.c	2023-06-13 23:08:10.000000000 +0800
+++ b/src/event/ngx_event_openssl.c	2023-06-14 15:43:05.834243714 +0800
@@ -1674,6 +1674,7 @@
 
     sc->buffer = ((flags & NGX_SSL_BUFFER) != 0);
     sc->buffer_size = ssl->buffer_size;
+    sc->dyn_rec = ssl->dyn_rec;
 
     sc->session_ctx = ssl->ctx;
 
@@ -2645,6 +2646,41 @@
 
     for ( ;; ) {
 
+        /* Dynamic record resizing:
+           We want the initial records to fit into one TCP segment
+           so we don't get TCP HoL blocking due to TCP Slow Start.
+           A connection always starts with small records, but after
+           a given amount of records sent, we make the records larger
+           to reduce header overhead.
+           After a connection has idled for a given timeout, begin
+           the process from the start. The actual parameters are
+           configurable. If dyn_rec_timeout is 0, we assume dyn_rec is off. */
+
+        if (c->ssl->dyn_rec.timeout > 0 ) {
+
+            if (ngx_current_msec - c->ssl->dyn_rec_last_write >
+                c->ssl->dyn_rec.timeout)
+            {
+                buf->end = buf->start + c->ssl->dyn_rec.size_lo;
+                c->ssl->dyn_rec_records_sent = 0;
+
+            } else {
+                if (c->ssl->dyn_rec_records_sent >
+                    c->ssl->dyn_rec.threshold * 2)
+                {
+                    buf->end = buf->start + c->ssl->buffer_size;
+
+                } else if (c->ssl->dyn_rec_records_sent >
+                           c->ssl->dyn_rec.threshold)
+                {
+                    buf->end = buf->start + c->ssl->dyn_rec.size_hi;
+
+                } else {
+                    buf->end = buf->start + c->ssl->dyn_rec.size_lo;
+                }
+            }
+        }
+
         while (in && buf->last < buf->end && send < limit) {
             if (in->buf->last_buf || in->buf->flush) {
                 flush = 1;
@@ -2784,6 +2820,9 @@
 
     if (n > 0) {
 
+        c->ssl->dyn_rec_records_sent++;
+        c->ssl->dyn_rec_last_write = ngx_current_msec;
+
         if (c->ssl->saved_read_handler) {
 
             c->read->handler = c->ssl->saved_read_handler;
diff --color -uNr a/src/event/ngx_event_openssl.h b/src/event/ngx_event_openssl.h
--- a/src/event/ngx_event_openssl.h	2023-06-13 23:08:10.000000000 +0800
+++ b/src/event/ngx_event_openssl.h	2023-06-14 15:43:05.834243714 +0800
@@ -86,10 +86,19 @@
 typedef struct ngx_ssl_ocsp_s  ngx_ssl_ocsp_t;
 
 
+typedef struct {
+    ngx_msec_t                  timeout;
+    ngx_uint_t                  threshold;
+    size_t                      size_lo;
+    size_t                      size_hi;
+} ngx_ssl_dyn_rec_t;
+
+
 struct ngx_ssl_s {
     SSL_CTX                    *ctx;
     ngx_log_t                  *log;
     size_t                      buffer_size;
+    ngx_ssl_dyn_rec_t           dyn_rec;
 };
 
 
@@ -128,6 +137,10 @@
     unsigned                    in_ocsp:1;
     unsigned                    early_preread:1;
     unsigned                    write_blocked:1;
+
+    ngx_ssl_dyn_rec_t           dyn_rec;
+    ngx_msec_t                  dyn_rec_last_write;
+    ngx_uint_t                  dyn_rec_records_sent;
 };
 
 
@@ -137,7 +150,7 @@
 #define NGX_SSL_DFLT_BUILTIN_SCACHE  -5
 
 
-#define NGX_SSL_MAX_SESSION_SIZE  4096
+#define NGX_SSL_MAX_SESSION_SIZE  16384
 
 typedef struct ngx_ssl_sess_id_s  ngx_ssl_sess_id_t;
 
diff --color -uNr a/src/http/modules/ngx_http_ssl_module.c b/src/http/modules/ngx_http_ssl_module.c
--- a/src/http/modules/ngx_http_ssl_module.c	2023-06-13 23:08:10.000000000 +0800
+++ b/src/http/modules/ngx_http_ssl_module.c	2023-06-14 15:43:05.834243714 +0800
@@ -290,6 +290,41 @@
       offsetof(ngx_http_ssl_srv_conf_t, reject_handshake),
       NULL },
 
+    { ngx_string("ssl_dyn_rec_enable"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_CONF_FLAG,
+      ngx_conf_set_flag_slot,
+      NGX_HTTP_SRV_CONF_OFFSET,
+      offsetof(ngx_http_ssl_srv_conf_t, dyn_rec_enable),
+      NULL },
+
+    { ngx_string("ssl_dyn_rec_timeout"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_CONF_FLAG,
+      ngx_conf_set_msec_slot,
+      NGX_HTTP_SRV_CONF_OFFSET,
+      offsetof(ngx_http_ssl_srv_conf_t, dyn_rec_timeout),
+      NULL },
+
+    { ngx_string("ssl_dyn_rec_size_lo"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_CONF_FLAG,
+      ngx_conf_set_size_slot,
+      NGX_HTTP_SRV_CONF_OFFSET,
+      offsetof(ngx_http_ssl_srv_conf_t, dyn_rec_size_lo),
+      NULL },
+
+    { ngx_string("ssl_dyn_rec_size_hi"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_CONF_FLAG,
+      ngx_conf_set_size_slot,
+      NGX_HTTP_SRV_CONF_OFFSET,
+      offsetof(ngx_http_ssl_srv_conf_t, dyn_rec_size_hi),
+      NULL },
+
+    { ngx_string("ssl_dyn_rec_threshold"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_CONF_FLAG,
+      ngx_conf_set_num_slot,
+      NGX_HTTP_SRV_CONF_OFFSET,
+      offsetof(ngx_http_ssl_srv_conf_t, dyn_rec_threshold),
+      NULL },
+
       ngx_null_command
 };
 
@@ -629,6 +664,11 @@
     sscf->ocsp_cache_zone = NGX_CONF_UNSET_PTR;
     sscf->stapling = NGX_CONF_UNSET;
     sscf->stapling_verify = NGX_CONF_UNSET;
+    sscf->dyn_rec_enable = NGX_CONF_UNSET;
+    sscf->dyn_rec_timeout = NGX_CONF_UNSET_MSEC;
+    sscf->dyn_rec_size_lo = NGX_CONF_UNSET_SIZE;
+    sscf->dyn_rec_size_hi = NGX_CONF_UNSET_SIZE;
+    sscf->dyn_rec_threshold = NGX_CONF_UNSET_UINT;
 
     return sscf;
 }
@@ -694,6 +734,20 @@
     ngx_conf_merge_str_value(conf->stapling_responder,
                          prev->stapling_responder, "");
 
+    ngx_conf_merge_value(conf->dyn_rec_enable, prev->dyn_rec_enable, 0);
+    ngx_conf_merge_msec_value(conf->dyn_rec_timeout, prev->dyn_rec_timeout,
+                             1000);
+    /* Default sizes for the dynamic record sizes are defined to fit maximal
+       TLS + IPv6 overhead in a single TCP segment for lo and 3 segments for hi:
+       1369 = 1500 - 40 (IP) - 20 (TCP) - 10 (Time) - 61 (Max TLS overhead) */
+    ngx_conf_merge_size_value(conf->dyn_rec_size_lo, prev->dyn_rec_size_lo,
+                             1369);
+    /* 4229 = (1500 - 40 - 20 - 10) * 3  - 61 */
+    ngx_conf_merge_size_value(conf->dyn_rec_size_hi, prev->dyn_rec_size_hi,
+                             4229);
+    ngx_conf_merge_uint_value(conf->dyn_rec_threshold, prev->dyn_rec_threshold,
+                             40);
+
     conf->ssl.log = cf->log;
 
     if (conf->certificates) {
@@ -890,6 +944,28 @@
         return NGX_CONF_ERROR;
     }
 
+    if (conf->dyn_rec_enable) {
+        conf->ssl.dyn_rec.timeout = conf->dyn_rec_timeout;
+        conf->ssl.dyn_rec.threshold = conf->dyn_rec_threshold;
+
+        if (conf->buffer_size > conf->dyn_rec_size_lo) {
+            conf->ssl.dyn_rec.size_lo = conf->dyn_rec_size_lo;
+
+        } else {
+            conf->ssl.dyn_rec.size_lo = conf->buffer_size;
+        }
+
+        if (conf->buffer_size > conf->dyn_rec_size_hi) {
+            conf->ssl.dyn_rec.size_hi = conf->dyn_rec_size_hi;
+
+        } else {
+            conf->ssl.dyn_rec.size_hi = conf->buffer_size;
+        }
+
+    } else {
+        conf->ssl.dyn_rec.timeout = 0;
+    }
+
     return NGX_CONF_OK;
 }
 
diff --color -uNr a/src/http/modules/ngx_http_ssl_module.h b/src/http/modules/ngx_http_ssl_module.h
--- a/src/http/modules/ngx_http_ssl_module.h	2023-06-13 23:08:10.000000000 +0800
+++ b/src/http/modules/ngx_http_ssl_module.h	2023-06-14 15:43:38.264102815 +0800
@@ -62,6 +62,12 @@
     ngx_flag_t                      stapling_verify;
     ngx_str_t                       stapling_file;
     ngx_str_t                       stapling_responder;
+
+    ngx_flag_t                      dyn_rec_enable;
+    ngx_msec_t                      dyn_rec_timeout;
+    size_t                          dyn_rec_size_lo;
+    size_t                          dyn_rec_size_hi;
+    ngx_uint_t                      dyn_rec_threshold;
 } ngx_http_ssl_srv_conf_t;
 
 
