I don't think they are slow, in that they just patch the way that namespace traversal is done and add a few namespaces... but they are definitely buggy; they will break CoreSessionTracking if you want to use more than one session id manager, among other things...
Hi! Transparent Folder ARE indeed slow. If you just use one or two and don't put much into them, it's fine. But we had sites with many nested TransparentFolders, and rendering the management screen (manage_main) sometimes took a couple of seconds instead of half a second or so with TransparentFolders removed. They make the system look up many more namespaces than necessary I guess. Stephan Richter wrote some "flatten" skript, so you can use TransparentFolders at design time and remove them all at once later. It's not a perfect solution however. We didn't have any problems with bugginess so far ... One solution I am personally working on is folders that just "register" or filter certain types of objects, so that you can have a folder (or tab) that shows all your SQL methods for example, though they actually are all in your root folder. Joachim