diff --git a/src/HttpServer.cc b/src/HttpServer.cc
index c7bc5b86..5665f7f5 100644
--- a/src/HttpServer.cc
+++ b/src/HttpServer.cc
@@ -367,7 +367,7 @@ int Http2Handler::on_read()
 int Http2Handler::on_write()
 {
   int rv;
-  uint8_t buf[4096];
+  uint8_t buf[16384];
   auto output = bufferevent_get_output(bev_);
   util::EvbufferBuffer evbbuf(output, buf, sizeof(buf));
 
diff --git a/src/h2load_http2_session.cc b/src/h2load_http2_session.cc
index b831d356..e2425a3b 100644
--- a/src/h2load_http2_session.cc
+++ b/src/h2load_http2_session.cc
@@ -170,7 +170,7 @@ ssize_t Http2Session::on_read()
 int Http2Session::on_write()
 {
   int rv;
-  uint8_t buf[4096];
+  uint8_t buf[16384];
   auto output = bufferevent_get_output(client_->bev);
   util::EvbufferBuffer evbbuf(output, buf, sizeof(buf));
   for(;;) {
diff --git a/src/h2load_spdy_session.cc b/src/h2load_spdy_session.cc
index f453d127..d61d4b8d 100644
--- a/src/h2load_spdy_session.cc
+++ b/src/h2load_spdy_session.cc
@@ -178,7 +178,7 @@ ssize_t SpdySession::on_read()
 int SpdySession::on_write()
 {
   int rv;
-  uint8_t buf[4096];
+  uint8_t buf[16384];
 
   sendbuf.reset(bufferevent_get_output(client_->bev), buf, sizeof(buf));
 
diff --git a/src/shrpx_http2_session.cc b/src/shrpx_http2_session.cc
index 0cfa116b..28dfaa90 100644
--- a/src/shrpx_http2_session.cc
+++ b/src/shrpx_http2_session.cc
@@ -1260,7 +1260,7 @@ int Http2Session::on_write()
 int Http2Session::send()
 {
   int rv;
-  uint8_t buf[4096];
+  uint8_t buf[16384];
   auto output = bufferevent_get_output(bev_);
   util::EvbufferBuffer evbbuf(output, buf, sizeof(buf));
   for(;;) {
diff --git a/src/shrpx_http2_upstream.cc b/src/shrpx_http2_upstream.cc
index c8335de5..e77d990f 100644
--- a/src/shrpx_http2_upstream.cc
+++ b/src/shrpx_http2_upstream.cc
@@ -566,7 +566,7 @@ int Http2Upstream::on_write()
 int Http2Upstream::send()
 {
   int rv;
-  uint8_t buf[4096];
+  uint8_t buf[16384];
   auto bev = handler_->get_bev();
   auto output = bufferevent_get_output(bev);
   util::EvbufferBuffer evbbuf(output, buf, sizeof(buf));
diff --git a/src/shrpx_spdy_upstream.cc b/src/shrpx_spdy_upstream.cc
index 8d70fd41..a16095de 100644
--- a/src/shrpx_spdy_upstream.cc
+++ b/src/shrpx_spdy_upstream.cc
@@ -459,7 +459,7 @@ int SpdyUpstream::on_write()
 int SpdyUpstream::send()
 {
   int rv = 0;
-  uint8_t buf[4096];
+  uint8_t buf[16384];
 
   sendbuf.reset(bufferevent_get_output(handler_->get_bev()), buf, sizeof(buf));