Do you checked the connection pool statistics of your datasource configuration?
If you have reached the 100 connections parallel the next thread/transaction/db-access must wait for a free one.
The possible solutions are:
reduce the duration of transaction where the database is used, that is only a small posibility because you need to read first and commit very late.
So the effort to optimize here is very high and depend to the application maybe not possible.
Other thing is to increase the number of parallel connections to the database, in that case you might reach the limit of one JBoss instance and you have to cluster the application.