Subject Trace cause of performance drop



I have a performance problem at a customer site.

At this point I suspect one specific external user connection to cause the problems, but I can't pinpoint it so exactly just yet. 

But there is one table, where there is inserted records almost all the time. 

When the user connects, does whatever he has to (this will cause reads and some inserts into various tables).

Then when the user completes the current routine, there will be updates of some 10-15 tables, which takes no time. 

Then an insert is done into one table. And the prepare of this insert can take up to 2 seconds. Normally it would be instantly.

The entire database is around 96Gb

Firebird is 2.5.

Running Classic on Windows

What will cause a prepare to become so slow?

It also seems as if the prepare is not evenly slow for all users.