Today I updated my iMac to OS X 10.11.2, in the hope that this might bring a solution to the problem plaguing us here. No luck. iTunes is just as sluggish with the new El Capitan version. My suspicion is that there is some problem updating the iTunes database, for after each change in iTunes (e.g. editing the album name or composer), I see that the timestamps on the iTunes Library.itl and .xml files are changed. And these updates take 5-10 minutes (with rotating beachball and iTunes reported as 'Not responding') each time.
The program is unusable.
I realise that Apple does not officially follow this forum (and have reported the problem over the Feedback path). I am usually a pretty patient chap but, after being unable to use iTunes since the end of October (luckily I have a Sonos system too and so can listen to my music without iTunes - that is also why I haven't reverted to an earlier version), I really expect some action or, at least, an explanation why the correction is taking so long.
Come on, Apple. Many users are suffering from this problem. DO SOMETHING!