Just some clips of undamaged Terminators moving slowly to prove Richard's tenebrous yet apparently stellar ( maybe it's the pretty gif? ) answer is incorrect:
In all of these scenes it would have been better for the individual Terminator if they had moved faster (such as the sixth link. The T-1000 could have easily caught up with John)
Of course, they can move quickly when they want to
Note how the T-1000 instantly breaks into a run here, dismantling xXGrizZ's argument that Terminators need to accelerate gradually.
But they move slowly in certain scenes because, obviously, it looks cool. It gives the impression that "It doesn't matter how slow they go, or how fast you run, you're still dead when they reach you." It's filmwork and I promise you you won't get an in-universe answer.
My guess at a possibly in-universe answer is possibly their Iridium Nuclear Power Cell. While it allows a T-800 to operate for several years at full power, suddenly pulling the fuel rods out to release more energy could weaken the cell's structure or be 'unstable' and so the Terminator only runs when it absolutely has to.
There is no in-universe information pertaining to the T-1000's power source, but it possibly has many microscopic versions of the T-800's power source that could be subject to the same stresses.