Possible reasons:
You're using an upatched version of 2003 Project and Server without the
latest service packs
You have a great many assignment records in this project
You're using a machine with limited RAM
You're saving across a busy or small bandwidth network with high latency
The plan is bloated and needs therapy
Take your pick and be sure to specify the version you're using in future
posts.
--
Gary L. Chefetz, MVP
MSProjectExperts
For Project Server Consulting:
http://www.msprojectexperts.com
For Project Server FAQS:
http://www.projectserverexperts.com