Hello avastnoob,
surely, avast! WebShield is an extra step in your downloads, so it is possible you will notice some delays. Especially if you create a large number of connections (hundreds) as some speed test do, you may observe some slowdowns - but transfer rates during continuous downloads should be limited only by the avast! engine power (and as the scanning during downloads is performed only in the memory, it is much faster than 20Mbps on today's systems). Judging from your log, your connections are reused successfully and several requests are handled on one HTTP connection - so I don't think there is anything strange happening that may indicate you have a problem.
How did you observe the slowdown? Is there a special file you are downloading where the slowdown is visible? Can you perform a similar test as Alanrf and post here the times and URL of the file your are testing?
Lukas