Slow network transfer speeds

We just added a WAP54G access point onto our linksys router.
Everything is setup and the wireless systems are online and on the
network but when I enable the WEP encryption it slows the internal file
transfers to a crawl.

My systems has a main server and all the computers access a database on
the main server through a mapped drive. The computers that are
wireless take forever to pull up information out of the database and
run the GUI program. To pull up the simpliest report takes about 5-10
minutes. On a wired computer it takes about 1 second.

If I remove the WEP encryption the file / data transfers are fine.

There are other security modes such as WPA Radius, WPA pre-Shared key,
and Radius. Should I try to use one of those features to increase the
speed?

My mode is mixed and SSID broadcast is enabled.

Thanks