Why is my iPhone getting slow after 17.3 update

My phone connection is very slow, ran speed test several times and I get various results, never over 75 Mbps. Ran the same test on my wife phone, hasn't been updated, and I got 295 mbps which is the aprox result that I was getting on my phone before the ios update. Is it possible to reverse the last update?

[Re-Titled by Moderator]