Does Unity's Dictionary performance depend on number of dictionaries?

I'm currently working on a personal project that is experiencing some odd performance issues regarding performance of generic dictionaries in C#.

My main issue is that a dictionary I'm using in the main thread is experiencing horrific performance (100-1000x slowdown) when there are dictionaries in other threads being used heavily. I'm wondering if anyone else has experienced this issue, and how they resolved it. Clearly there are some odd things going on in the background regarding Mono and threading, but I don't know what.