Might try shutting down and restarting.
Any pattern on where/when it's slow (eg. slow at coffee shop wifi but ok at home)? Just a specific website site?
Poor wifi signal.
Poor cellular network.
Site is slow.
Using a slow DNS server on wifi. Can change that to point to Google's: Settings > Wifi > circle-i (aka Info). Tap Configure DNS, then Manual. Delete the entries there, then add in (no quotes) "18.104.22.168" and "22.214.171.124". Or can add Cloudflare's server, "126.96.36.199", or, IBM's "188.8.131.52".
Nuclear option, can do a reset of Network Settings and start that fresh.