I have had a copper IUD (paragard) for over 5 years, and have generally run 50 mpw (sometimes up to 80) during this time.
Prior to the IUD, I had bad cramping during my period which was completely alleviated with advil and a normal (not heavy) flow.
With the IUD, I have the same amount of cramping during my period (again, reduced to zero with advil) and the same total amount of flow, but concentrated more on one day. But the heavy flow day is not so bad: I can still get a good few hours out of a tampon. I imagine for women who had a really heavy flow prior to the IUD, it might end up being unbearable with an IUD, but that experience is far from universal.
For me personally, my running improved after the IUD because hormonal birth control was making me gain weight (along with every other side effect listed on the back of the package). Again, this incompatibility with hormonal birth control is something that is also not universal among all women.
The copper IUD has some key advantages (no hormones; works for 10 years) over other options, so I would recommend it to most women (and don't complain about cramps until you've at least tried to pop some advil) except for those whose menstrual flow is much heavier than average.