You do realize that Photopea is JS running in your browser right, aka it’s slow.

Yes I realise it's running in a browser.

No it's not slow.

It's running 5mb of web assembly, so unless running regular photoshop is using more than 50% of your CPU, you're not going to notice any slowdowns.

The only time you're going to notice a slow down is if the thing you're doing would already be annoying slow in photoshop (e.g. opening >500mb non-raw files, in which case it being slow in a browser wouldn't matter since you'd be better off processing the files outside of photoshop anyway).