I am syncing the values with delta time. My theory was that the lag was causing the acceleration values to be incremented by a lot more than they should, then subsequently incrementing the velocity by that much. I'll just have to check and see.
EDIT: Yeah, I changed a bunch of physics around and even ran a "lag machine" and I can't reproduce the bug. Technically, you're supposed to be able to jump higher by holding the jump than just tapping it, but PurpleMetro's screenshot shows a higher jump height than what should be possible.
Well, one of my old Love2D games I originally made on Linux, ran normally. When I play-tested it on Windows however, the movements were totally fast and so unsynced, even though I use deltatime so, yeah.
Can't Love2D detect which operating system is being used by the user? If so then make it set movement speeds based on which OS is being used?