Performance due to large files or large views is a real issue. There have been numerous discussions about things that can be done to improve perf. I suggest doing a Google Groups search for InfoPath Performance.

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Greg Collins [InfoPath MVP]
Visit http://www.InfoPathDev.com
Hello,
we use a relatively large infopath file which has the size of 1.5MB. There
are alot of dropdowns with regarding tables in it. In use, the formular is
extremely slow and needs alot system performance. The performance meter in
the task manager always climbs to 100% when we start this document or even
using a dropdown.
Is there any chance to boost this performance like in access repair database
or something?
kind regards
Ralf Erdmann