When reports refresh on the dashboard there is a visible flicker of the report.
This is so you can confirm the report has actually refreshed, as sometimes there won't be any changes to the data.

A grey overlay is displayed during the refresh which can be disabled from Configuration > Page Format





Note:
-This is only available in 5.2 (July 2011 build and later)
-Tables are charts are generated diffently, so you may still see a flicker when using charts. Charts are actual images that need to be regenerated each time.