vaadin7 run slowly on IE8,how to improve?

Vaadin 6 / 7 / 8
1

any method to improve the performance?

2

Yes, update your browser. IE8-10 are no longer supported.
http://venturebeat.com/2016/01/12/microsoft-ends-support-for-ie8-ie9-ie10-and-windows-8/

If you are really stuck with a security nightmare (i.e. an unsupported browser) then you should consider not deeply nesting your layouts for improved performance. Use CssLayout over HorizontalLayout/VerticalLayout wherever you can.

HTH,
Enver