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?
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?