Did you use aggregate functions? The Simple Chart is a terrible widget when it comes to performance if your Y-value is a sum/count/... over hundreds of y-objects.
Just use firebug and you'll see the problem. All Y-objects are retrieved and aggregation is done client-side. The client has to loop through hundreds or thousands of objects to determine the aggregated value.
So my advice: If you care about performance never put unaggregated data into the SimpleChart widget. Instead: aggregate server side by using OQL and a helper domain object where you store the X and the aggregated Y value and connect the simple chart widget to that object.
This should be in the known issues of the widget because it can give issues on both client and server/database side.