The file system isn't slow. The slowness will be present in any file system due to the file system filters that all file system calls pass though.

Right, by “file system” here I mean all of the layers between the application talking in terms of named files and whatever first starts talking in terms of block addresses.

Also, as far as my (very limited) understanding goes, there are more architectural performance problems than just filters (and, to me, filters don’t necessarily sound like performance bankruptcy, provided the filter in question isn’t mandatory, un-removable Microsoft Defender). I seem to remember that path parsing is accomplished in NT by each handler chopping off the initial portion that it understands and passing the remaining suffix to the next one as an uninterpreted string (cf. COM monikers), unlike Unix where the slash-separated list is baked into the architecture, and the former design makes it much harder to have (what Unix calls) a “dentry cache” that would allow the kernel to look up meanings of popular names without going through the filesystem(s).