Look children let me explain something to you guys.
I have been in TKD for over 10 years now. I started when I was 13 and I am now 24. I have competed on the City, State, National, and International levels. The crap you guys are speaking of all has adv's and disadv's.
Look if we are agreeing that the other person is faster than you - now if you are the same height you spar primarily definsively. Keep your distance with front leg sidekicks and 45 degree pivots (like then he comes at you with a hop-step kick or just runs in), evading is very useful and conseves your energy. If he is persistant wait until he is tired and counter him usually with a turning back side kick or turing hook kick then hop-step toward him and contnue throwing combos until he is forced out of the ring. If he is quicker and gets in close then use a pushing font kick - if he falls down you are awarded one point.
If the guys is taller than you and faster then you need to work on an agressive approach where you get in and out quick. Probably fast cuts to the inside (watch your head), use simole kicks no hook kicks, only your fastest and accurate kicks like fron side, turning back, and roundhouse. Use your front leg followed by your back. Try to keep him off his rythem since he is faster.
If you are taller (and he s still fast like ha been the case the whole scenario) - spar defensivly keep him at a distance like the first scenario i talked about. This time let him inside and go for the crescent and hook kicks to the head. When and if they tire out then move to an offensive attack.
No matter what situation you are in while sparring you always have to be adaptive to the situation - this is why we have continuous sparring. For the longest time back when I was competing I weighed 187 pounds and was put in the heavyweight divisino becasue i was stockier and bigger than most guys in whatever divisinon I was put in back then (cant remember). I have always sparred people A LOT bigger than me. However I was always faster. In fact I am probably one of the fastest heavy weights in college competition right now. I remember last year I sparred in the Carolina Open here in Chapel Hill. I was 194 pounds. There was only 1 other person in the heavy weight division he was 240 pounds and 3 inches taller. Even though I was faster I lost because 1. I lost a lot of speed. 2. I was not prepared to spar and pulled a groin muscle the night before 3. his counters were right on the money 4. it was like mike tyson sparring george forman. Sometimes there just isnt anything you can do, but you can always put your heart into the match and earn the respect of your competitors. Now i weigh 225, have almost NO speed but I am as big as the guys I will be sparring - we will see whos timing is better on counters, but I am making every effort to start training 3 hours a day now and get my speed back. I remember when my back leg roundhouse was faster than an avg persons front hand jab.
I hope this helps you out.