iTunes 12.2 very slow - beach ball spinning all the time

Hello everyone. I have a problem with iTunes that leaves me desperate and I would be glad for any feedback that might help me solve it.

I am using iTunes version 12.2 on an iMac (3.2 GHz Intel Core i5 (late 2012), 8 GB 1600 MHz DDR3 RAM), latest Yosemite version 10.10.4.

I have a large iTunes library that has grown over more than 10 years. It consists of approx. 50.000 titles, size is 390 GB. Most songs are mpeg, some AAC, quality ranges between 256 and 320 kBit/s. Over the years I digitalized many vinyl records and ripped my complete CD collection.

All my music is organized in playlists (approx. 5.000). All data are on the internal disc.

After upgrading to 12.2. iTunes became incredibly slow (It was slow before, but not as slow as now). Every time I click to play a song, the "beach ball" is spinning for 30 seconds. Same happens when one song is finished and the next one is about to begin (without me clicking on it). During the beach ball spin no action is possible. I can't even skip through an album or playlist any more.

Same problem on my office Mac (MacPro 2014, Yosemite 10.10.4, iTunes 12.2, a copy of my library running there).

I already tried the following trouble-shooting, with no results:

- started iTunes in safe mode (although no plug-ins installed)

- deleted "previous itunes libraries":

- emptied caches

Thanks for help.