Advance billing - AR

In the Account Receivable module, I'm doing advance billing and
I'musing currency base (my base currency is in CDN dollars, so the bill
is taxable). When I add a billing line and write miscellaneous charges
in this line, the delay is really long when I come out of the
miscellaneous window. When I click on close, we have to wait up to half
an hour before being able to access the bill, and the window freezes.
Do you have an idea why it would do this?