When I enable extensions in Safari 5.1.1 on Snow Leopard I get severe keystroke lag. When I disable it the lag goes away. Lately this showed up while testing the Feedly extension.
It's not a new problem ...
- Safari Web Content high CPU usage: Apple Support Communities
- Web Process + Safari now hogging 50-75%...: Apple Support Communities
I suspect it's worse in Snow Leopard in Lion; my now most 5.1.1 users are probably on Lion.