The movement of disk read-write heads makes it the slowest part of a computer system.
Even if you don't use #temptables, the system is using the tempdb for indexing, sorting, snapshot isolation and more.... So you want to add more read/write heads.... to keep the system humming...
That is the reason that you should have for high performance:
1. Tran log on dedicated RAID 1 (or 10)
2. Tempdb on dedicated RAID 1 (or 10)
3. Frequently used indexes on dedicated RAID 1 ( or 10)
4. Data on dedicated RAID 5 (or 10)
SQL Server 2005 Performance Tuning Training - http://www.sqlusa.com/order2005highperformance/