Bad advice and contrary to what both Adobe spokespeople and independent "gurus" say, as well as pros with catalogs of over 500,000 photos who report no slowing. And it is just not the way LR is designed to work. You can only open one catalog at a time and there are no actions that can be performed across multiple catalogs. What if I want to see all the photos of waterfalls that I've shot in the last ten years? I'm supposed to open 120 catalogs, one after another? And if I want to see all those shots together in one thumbnail grid, I do what? Make yet another catalog called Waterfalls and whenever I find some waterfall shots export them as a catalog and then import that catalog into the Waterfalls catalog? Hours of work instead of the seconds it would take to click All Photos > Keywords > Waterfalls.
If your LR is crawling, look for the reason elsewhere.