Slow Lan Performance on a Switched netwo

Recently we put up 8 Catalyst 2900 XL switches to replace our old hubs. We disabled Spanning Tree protocol on all switches except the one connecting our servers. After the change our LAN performance slowed down to a crawl. WE are using a Netware 4.11 server with 2 NICS. All out machines use 3COM cards. We configured all workstation ports to half duplex 10MBps. After this, the speed improved but its still not as fast as before.
What are we doing wrong?
Thanks for your help.