No, mIRC doesn't have anything that can limit download speed... even if you want it to (at least, not unless you have a script that does it).
You can have pdcc and fast send on and your packet size set to 4096 and they could help, but usually it is minor. Like I said, there isn't really anything you can do about it other than try other servers. I've seen people download at 100+kB/s from one fast source and 10kB/s from another fast source. It depends on a lot of factors and it's usually not something you can change.
