Files/folder list slow to display after playback

I have a WDTV Live HD with 2-2tb drives and the latest firmware…  Both drives are within 30-50GB of capacity, mostly with .iso files.   After playback, if  press Home, then select Folders, it may take 10 seconds to display USB1 and USB2.  If after playback I page up and down in list mode, it will soon freeze for about 10 seconds, then scrolling and paging resumes normally.  It is as though the directory of the drives is being updated, but now slowly.  The problem didn’t arise until the drives were >90% full.  Before that time the file/folder list displayed almost instantly.  Is there some workaround that might fix this, such as defragmenting?  Thanks.

You said it yourself, a defrag will help!! Still… Most drives suffer mayor slowdowns when nearly full, both internal and external drives…

Is there no way to clean up the space? I mean, I know there’s a random moment when you feel like watching a specific movie or episode and it has to be available, but that takes space off the drive… Along with performance.