A File format should not in any way make it run slower.
But OCR absolutely will, Making the document text searchable takes vastly more processing power (and TIFF couldn't have that enabled) So verify that it is not turned on in the default template (or the one you are using) via CWIS > Properties > Services > Network Scanning > Default Template
File Size very well could be an issue, and if that is the case I would think cache size is at fault, and since you can't adjust the cache, you would need to make sure the place reserved for it is not full/filling. So the first thing to do would be to make sure people are not storing files on the printer for later retrieval (Scan to mailbox). So check in CWIS and delete anything un-needed in CWIS > Scans > Folders
