I own a 2016 MacBook Pro and a custom built Windows gaming machine with an i7-7700, GTX 1080ti, 32 GB DDR4 RAM, and a 1TB SSD and on both systems iTunes lags terribly. I sometimes can't even drag the window around without slideshow rendering. Updating takes 2-3 minutes and sometimes requires a restart (for a media app!). It's time to rewrite - maybe this is the start of that?