I'm still trying to get to the bottom of it, but it looks like the intent was to implement DNC functionality. Calling any number on the list results in a classification of "Blocked", which will prevent manually dialed calls, but will not have any effect on Dialer! This, of course, means it isn't really feasible to break it up.
I can think of a number of alternative approaches that would be preferable for various reasons, but before I go shooting my mouth off, I wanted to see if it was exceeding any kind of built-in limit and / or recommendation.
I will do some load testing to see if I can figure out the impact, I just was hoping for a starting point - if you had come back and said "yeah, more than 100 and you're screwed!", it would "adjust" priorities somewhat! Given your comments about 10,000 be a limit in a previous version, I'm going to put this on the "we need to look into this further" list ;-)