Start by downloading the older version of iTunes: http://mac.filehorse.com/download-itunes/5833/
I got the link from another thread on this board, and it works.
I'm sorry, but while this is genuinely helpful in that it may likely temporarily fix the frustrating symptoms that many people are seeing with iTunes 12.3.1, and I and others are indeed grateful for the sleuthing and sharing going into helping others make iTunes at a minimum usable, it all stems from a real breakdown in responsibility and accountability from Apple and desperately requires a prompt fix by Apple. Despite a known major problem with this latest release, the refusal of Apple to acknowledge this issue and fix it means that it has now driven its users to have to download versions of Apple software from unverified sources, delete files via Terminal (many evidently for the first time), install the unverified, non-current version of critical software and hoping nothing breaks during the downgrade, and then disable auto-update to prevent both a) the sure return of a known, verified, repeatable, major bug, and b) any ability to take advantage of future iTunes updates if a real fix isn't addressed and incorporated in the iTunes code base going forward.
Any responsible, responsive developer would acknowledge this ASAP, and then fix it ASAP (I can't count the number of times I've seen small developers post an update on the iOS or Mac App Stores, and before I ever get around to downloading it, they've already posted a note saying there's a bug in that new version and please hold off on downloading, and then, by that afternoon, boom, there's a fix) - Apple's refusal to confirm known problems is evidently causing many of its longtime users to suffer from the gaslight effect ("maybe it's something I've done", "maybe it's something wrong with my library/machine", "maybe it's my responsibility to fix this") and waste countless hours going in circles with tech support, and its inability to fix these known problems is leading its users into risky, convoluted, unsupported dead-end-downgrade-fork solutions.
Fix this, Apple. I know the bulk of your profits now come from iPhone sales, but the only reason I've bought piles of iPhones over the last 8 years is because they work with iTunes. Now wake up, shape up, fix this... and apologize, willya?
Note: for anyone just following this thread, there is also a much-longer thread on this issue here:
Itunes not responding, running extremely slow
And others:
My iTunes is non responsive with iTunes 12.3.1 I get that coloured beach ball constantly.
iTunes 12.3.1 Memory Hogging
After upgrading to 12.3.1, iTunes slowed way down, the rainbow appearing with every command, every click. Support could…
On getting by with multiple bugs in iTunes 12.3.1 update (some problems and fixes compiled)
Itunes 12.3.1 not responding on 10.10.5
etc...