slowdown java application

I have written an application in java that use a database on a Linux SapDb
Server 7.3. My customer uses some clients with Win Xp. After a while the
client disconnected from the database. I changed the session timout to 0
with Database Manager (from a client connected to server) and the problem
disappeared.
Now all is ok but, if the client application is iconized for a long time,
when it is restored it is very slow. Is it a problem of jdbc driver? I use
sapdbc.jar of size 339.794 and JRE 1.4.1.
Best regards.