this is an out-of-left-field theory, but it might explain things.
how's your PC, specifically graphics card? mine explosions throw alot of Shrapnel and Smoke, which the game actually spends alot of processing resources on. i'm wondering if your Sim Speed spiked downward because of the mine graphics. it would certainly make it look like you were moving extra slow (everything else would be super slow as well though).
other than that i can't think of anything that would create the appearance of being slowed by a huge amount with no debuff icons to explain it.