How can I resolve slow internet speeds on iPhone 12?

I’ve recently updated to ios 18.4.1 and I have problems with the internet. Things download painfully slow, an application that used to download in 10 seconds is now downloading in 2 minutes. Videos tend to stop at some point and when i fast forward them it stops to load a little when on the previous version it would work fine. It feels like my internet aged 30 years in 10 minutes. I’ve tried on other phones (not iphones) and it works just fine. Is it from the update or is it just my internet?

[Re-Titled by Moderator]