** This issue is addressed in ArcGIS 9.0 Service Pack 2 (SP2) **
Drawing after performing a join may be slower due to the processing of the joined data. The slower drawing should only occur, however, if conditions such as symbology or defintion queries have been applied that involve joined fields. If it has not, the layer should draw just as fast as if it were not joined.
This bug addresses an issue where the slower drawing occurs even when not using the joined fields.