Greedy kernel extension
Q According to Activity Monitor, in macOS 10.14.6 the kextd kernel extension is using over 8GB of memory and slowing my Mac down badly. How can I diagnose and fix this? A
The kextd kernel extension manages the loading and unloading of other kernel extensions, which shouldn’t result in it using large amounts of memory itself. There doesn’t appear to be a known bug in that version of macOS Mojave, and even if there were, it’s well over two years since Apple last fixed any bug other than security vulnerabilities in that old version of macOS.
The cause of your current problem is therefore most probably because the system itself has become damaged, or it’s the result of a third-party kernel extension installed on your Mac. As this has occurred relatively recently, it’s worth looking back carefully to discover whether you might have installed or updated anything, particularly software containing a kernel extension. Check those listed in the /Library/Extensions folder for anything bearing a suspicious date.
When removing third-party kernel extensions, it’s best if possible to use the app or original software installer which you suspect may be responsible. If you can’t do that or resolve the problem in any other way, then you’re going to need to perform a clean reinstall of Mojave, and to apply subsequent security updates.