The limit on a single catalogue is 128000 items, or a 2Gb ivc file , though effectively that's 1.5Gb. To get the most images into the catalogue, don't set the import previews option, and set the thumbnails size to something like 320 pixels and at medium quality.
iView is there to organize and control your picture collection. Breaking it into smaller catalogues is breaking up that organization and control, so you'll inevitably hit issues just like you are doing now. If it slows down a bit, it's a small price to pay - and less than the time you'll waste trying to keep multiple catalogues in sync.