Comment 1 for bug 890221

Onboard from 0.96.0 tries harder to only draw what's really necessary, ie. often only the pressed key and its neighbours. Is this with a fresh version or still 0.95.0?

Then there is key style "Flat", which was meant to be a fall-back on slow systems. There are no gradients or fake emboss effects to slow it down. In Preferences->Themes->Customize theme try setting the key style to Flat. Does that help?