In the beginning, iTunes never worked well once you had 10k+ songs. The user flows weren't designed to navigate large libraries, and it also slowed to a crawl.
And it always felt like straight up malware on Windows with all the QuickTime update prompts.
My library was never that big, but I knew people who had ones at least that size and didn't have any complaints, although given your last sentence it may be relevant to note they were all Mac users. The three-column browser view seemed to work very well for "drilling down" in collections, at least for me. (That browser's actually still in Music, even though it's unnecessarily hidden under the Songs view.)
> It always felt like straight up malware on Windows with all the QuickTime update prompts.
I'm not positive QuickTime was the source of an awful lot of Windows iTunes' badness, but I have strong suspicions. :)
The three pane view was pretty good for my large-ish library (~15k songs) and I didn’t have stability issues. But I wasn‘t there in the beginning. I guess it had at least a good stretch somewhere in the middle.
And it always felt like straight up malware on Windows with all the QuickTime update prompts.