My iTunes is non responsive with iTunes 12.3.1 I get that coloured beach ball constantly.

Like why did it just become so agonizingly, painfully, horrifically laggy and slow after this last update?

I wish I knew.

I have the exact same problem as both of you. I have fiber optic broadband (tested with the provider, performing as, if not better than expected) and it's taking 7 or 8 minutes to download a 64mb app update. Before iTunes V12 , it downloaded an app update of that size in about 20 seconds. In fact, iTunes has become incredibly laggy with a lot of things. Even just clicking on 'My Music' results in the spinning colour ball for several seconds. It's almost become unusable. It's much quicker to download app updates over wi-fi than it is online. And something seems to have happened with external server support. I keep all my music on a network server and it worked beautifully when iTunes 12 first came out but both recent updates have caused that to become very slow as well.