Intelligent playlist lag when sorting by Key

Problem:
Sorting by key within an intelligent playlist causes slowdowns proportional to the size of the playlist.

Details:
Sorting an already generated intelligent playlist by key, or generating a new one when already set to sort by key, is much slower than sorting by any other attribute (artist, genre, etc.). It seems to be tied to the number of tracks in the generated playlist - for example if it's only about 100 tracks the lag is minimal, though still noticeable relative to other sorting options. 500 tracks causes a lag of about a second before displaying the playlist but the waveforms will continue moving during the lag. Occasionally it will lock up completely for a split second before displaying the playlist. 1000 tracks (which I understand is the maximum number per playlist) causes the entire application to become unresponsive (waveforms freeze, windows Not Responding text, blue spinning wheel at the cursor) for a second or two.

This ONLY occurs in Intelligent playlists. Normal playlists can be sorted by key instantly, and I can also sort my entire library (approx. 8500 tracks) by key instantly too.

I've replicated this behaviour back in version 5 and every version of 6.x.x. I'm running Windows 10 64-bit, on an i7 8700K and all of the files are on an SSD so there's no shortage of horsepower (issue also occurred with the files previously stored on a regular HDD).