Windows Defender firewall is fine, it does not slow your internet speed. I can confirm this, as a I have a 12 year old Core 2 Duo laptop running Windows 10 v2004, and I'm getting ~100 Mbps d/l speed on it (802.11g).

I have tried Bitdefender AV free (which claims very low system resources) on the C2D laptop, but it actually does result in a noticeable lag and high system resource usage.

This is not the case using just the latest Defender and built-in firewall ...