Amethyst can get sluggish when there's a lot of spam posts, turning off some global feeds to some relays helps a lot.

I'd Play around toggling the up and down arrows and globe icon from the relay page until you see a global feed you find useful 🤓remember you can add paid relays to download events even if you haven't paid to be able to post on them

I'm tried Amethyst on a modern flagship and also a on 5 year old budget smartphone, both snappy after some tinkering 👍