Assuming the code has no checks for 32/64 process.
I have a pre-compiled serializer dll that reads objects from a stream. The same deserialization dll runs 10x slower when run in 64 bit customer environment but I can't reproduce it on my machine. In 32 bit process it runs fast both in my and the customer machines.
A profiler shows much time spent in GC but the memory snapshots are fine (64 is 1.5-2x bigger than 32 but nothing notable).
What should I look for?