Because Imported Items are accessing the file system on your computer, (as opposed to the database files that are cached into memory) things that affect disk I/O are a big deal.
From a maintenance perspective, you can check to see if your disk needs defrag'ed, as that can slow performance.
One client practice has ~65G of imported items, and we are testing out RAID1 of solid state drives(SSDs) for the imported items.
