Which node in the Media Tree are you using? Is it Folders or a Collection node like Music?

Have you tried a rebuild of your data base. Sometimes this help get all the indexes up to date File->Manage Database

I'm running MM on an IBM Thinkpad 425. My library holds 3000 files. It takes very long until the pictures are displayed, even when I just scroll forward. It looks like MM is always fetching information directly from the database.