It was very slow before, but browsers improved performance a lot, multiple frameworks use proxies for their reactive primitives under the hood, Vue, Mobx and so on. SolidJS is one of the fastests frameworks out there and is built mainly on proxies, so proxies themselves can't be that slow.

Granted in very very tight loop regular object get/set will be faster, but performance is more than good enough today. If you have an use case for it, use it.