Well, if your framerate is like 60fps...your enemies are harder than a person who's running at 30fps or lower.
I remember playing Quake 2 deathmatches against my older brother back in the day. He was on broadband, I was on dial-up. I'd see some health or ammo and head toward it, no sign of my brother. Just as I got to the health, I'd see him zip by me, there'd be a pause, and then I'd be respawning. I cursed my brother alot in those days... I know it's not the same as what you're talking about, but I just thought I'd share another of my MP experiences.
When I started up the game last night, I set it up for 'unknown enemies' and 'unknown location' for the xp bonuses, thinking I'd level up a few times and start unlocking things. I got reapers both times, and the "N7: Cerberus Attack" (on Tuchanka) map the first time, and the Benning map the second. They flanked me within seconds on both maps.