msquic: Performance problems with 1.8 when SendBufferingEnabled is FALSE

We are sending video data over MSQUIC. In order to get the best possible performance we allow multiple simultaneous network sends to occur and try to keep about 8 video frames in flight when at all possible. It is very common that we are receiving and sending data at the same time across many streams.

We have found that when we send with SendBufferingEnabled = TRUE, then the performance on all sending is great and that calls to StreamSend() typically take <1ms to complete.

When we send with SendBufferingEnabled = FALSE, then the average time taken to send always seems to be about ~41ms in many cases (but not all, some are very fast).

It seems that this value of ~41ms is the same across many machines and when sending either large (150 kbytes) or small packets (1000 bytes). This is with 8 pending sends at any time.

We see the same problem when sending both to localhost or to another machine (which rules out network performance).

Using msquic v1.4.0.0 did not show this problem, using version v1.8.0.0 does show it.