Apparently the fire rate which is used to to fire buffered shots is wrong (way too low) in this case for some reason. Need to check what's going on there. Thanks for the nice videos which show the problem very clearly.
Luckily the original fire rate doesn't seem to be throttled by the system.
btw: Shot buffering isn't caused by high pings (as long as they are constantly high) but by ping or FPS jitter (quickly changing latency and/or ping), micro lags or packet loss.