Just registered to chime in on this issue as well, since I'm experiencing the same behaviour: Constant CPU utilisation of around 30% from ESET with WSL running.
Instead of excluding the entire "%userprofile%\AppData\Local\Packages" directory, I'm working around this by only excluding "%userprofile%\AppData\Local\Packages\CanonicalGroupLimited.Ubuntu18.04onWindows_79rhkp1fndgsc\LocalState\rootfs". Admittedly this is still not exactly ideal, but it's a slightly more targeted approach at least. It might be interesting to further restrict exclusion rules to hone in on specifically "offensive" files/directories. (It's not just "bin", since only excluding that still leads to high CPU usage.)