For example, my understanding is BIDS Slowly Changing Dimension transformation is a performance hound for large tables.
If your finding the standard SCD component of SQL slow, have a look at Todd McDermitts Kimball SCD component.
I've switched to using his Kimball SCD for a number of reasons. First of all it's faster. As mentioned in the details, it doesn't destroy the dataflow when you need to make changes. Also, it provide a wealth of auditing information. Well worth a look!
You can find it here: http://kimballscd.codeplex.com/
MCITP: Business Intelligence Developer (2005)