Thank you for your post.
What version of FileMaker Server 12 are you using? What version of Mac OS X are you using?
If you run Activity Monitor on the server, do you see any process that is using a lot of memory? If so, which process?
Does the issue only occur with one field or with all fields?
Any other information you can provide would be helpful in narrowing down the possible causes.
find for the balance due > 0 will not produce the correct records.
So this is then a case of the find not finding the correct records are the calculation not computing the correct result?
A review of bug reports referencing "find" in the Known Bugs List refers to a number of issues associated with finds, hosted files and even the inequality operator.