Hi Mika5499,
How fast an internet connection will be will depend on the capabilities and design of the network driver in use. Avast uses the one indicated, and it would likely have to be upgraded to run at 1gbits/s. As things stand now, avast SecureLine TAP Adapter v3 is what it is.
Which, is likely why both you and the OP, are reporting a 50% drop in connection speed over non-VPN network connections.
But that information is missing in physicsboy's original post. Because of that missing information, rather difficult to pinpoint the cause without it.
I'm using an older system where the max speed is 100mbits/s, so the drop in speed is really not noticeable, but it is still there. If you are running 1gbits/s, and running a faster network setup than I have, then you would see a greater drop than I.