The most common causes of this are:
* Anti-virus
* Thumbnail building
Please read:
http://wiki.jriver.com/index.php/Troubleshooting_GuideYou can check what percentage of your Thumbnails have been built in the output from the
Reporter function mentioned in the System Requirements section of the guide.
If you don't have a very high percentage of your Thumbnails built, then performance will be impacted as you move around Views to sections where no thumbnails have been built yet. That sounds like the case here.
This problem will be severely exacerbated if your Anti-Virus application is interfering with MC's process, or forcing a scan on your media files before each is available for MC to build the thumbnails on the fly. There are instructions within the guide for taming your Anti-Virus, with a couple examples (even if these don't match your particular application, the concepts remain the same).
If this is the case for you, and the percentage of thumbnails built is low, I'd recommend running the Thumbnails builder manually overnight. This is found at
Tools > Options > Tree & View > Thumbnails > Build missing thumbnails. Hit that and get it started, and then go to bed.
If performance is still slow, and you have done that, or you already have a high-percentage of thumbnails built in the Reporter function, then I'd look squarely at AV. But read through the rest of the guide, because there are other potential causes addressed there.