If you need really low interrupt latency you should go for installable interrupt service routines (ISR). On the PXA270 we reach latency times as less as to 10-15 us.
Delays on the T20 can vary. With disabled Dynamic Frequency Switching (DFS) running on full speed and low workload ISRs get down to 2.5-5us latency time where as ISTs get down to 10-15us. With DFS enabled ISR latency time can go up to 100us and IST latency up to 250us.
Latency times on WinCE 6 and WEC 7 are about the same.
See also the Win CE help for more information.