message: Reserve space in frames vector.
  Summary: 2 actually seems to be good enough for the majority case that's expensive (heavy stream frame writing).
diff: @@ -15,6 +15,10 @@ namespace {
  // maximum length of packet length.
  constexpr auto kMaxPacketLenSize = sizeof(uint16_t);
  +// Amount of space to reserve in the quicFrames_ of the builder. 2 seems to be
  +// good for most cases, as the heaviest path is usually 1 stream frame per
  +// packet.
  +constexpr auto kPreAllocatedFramesSize = 2;
  } // namespace
  namespace quic {
  @@ -130,6 +134,7 @@ RegularQuicPacketBuilder::RegularQuicPacketBuilder(
  headerAppender_(&header_, kLongHeaderHeaderSize),
  bodyAppender_(&outputQueue_, kAppenderGrowthSize),
  version_(version) {
  + quicFrames_.reserve(kPreAllocatedFramesSize);
  writeHeaderBytes(largestAckedPacketNum);
  }
  