When I originally signed up with a limit that was 25% of the current limit it took about a week to get not quite 1/2 my 21,000 tracks. At that point nothing seemed to be happening so I rebooted and got all but about 300 tracks. Many of those 300 tracks were old rips and weren’t high enough quality for IM to mess with. I selected each one and had iTunes re-rip them with a higher bit rate and eventually those were also matched. So to get about 21,000 tracks took a bit more than 2 weeks, a couple reboots, and some massaging of tunes.
When Apple increased the limit I rejoined my two catalogs (I separated popular music and my favorite jazz/big band from the rest) giving me not quite 60,000 tracks. Once again it synced a large group and stalled…rinse and repeat…I now have everything synced but I now am in the process of verifying that iTunes actually properly matched. Originally I had a largish number of tracks that IM fumbled on. In some cases clean versions were subbed for not clean, sometimes it was the reverse. And then there were the huge flubs like 1/2 a CD of 60s folk getting matched with hip hop. Not a happy surprise😁