The issue I am facing:
I am experiencing significant reduction in speed (upload and download). It is less than 6Mbps
if I disable blocking and check the speed, it goes to 30Mbps +
it really hurts, when using Microsoft Teams
Pi-hole has no impact on your internet speed. It is a DNS server only. Once a client has resolved a domain name to an IP, subsequent traffic between the client and the IP is directly between the LAN client and the router, with no involvement from Pi-hole.
You may be seeing an inaccurate speed test result, depending on which test server is being used. If your upstream DNS is not location aware, you could be getting a non-local IP that is slowing the test.
You could also be getting a distant IP for the teams software.