None of the above.
I suggest another solution: encouraging players to defend and discouraging players from trying to capture flags alone by adequate changes to the scoring systems.
As [-z-] noted before, we should treat the root of the problem, not its symptoms, that is, players own behaviour.
First I would like to make something absolutely clear: there is no one fast FC who can break a good coordinated defence. Even one good player on defence that uses the terrain and a variety of weapons to his advantage can put down any flag carrier. Your movements as a FC are always predicted, there is one place you want to reach and one general direction you would be heading.
I play virtually (literally?) all my games on CTF servers, the public kind. No fuzzy warm cooperation there like on the private servers. Most players, and everyone at some time of the game, are out for the highest personal score. This human weakness is ougt to be manipulated. Perhaps I am wrong or everyone else is blind, but the current scoring system is in need of a change. Give more cookies to players who guard the flag, either by retrieving the flag or killing the flag carrier.
Today it would still be beneficial to a caper to miss 3 caps but score one: 20 > 15.
All you have to do is instill fear into the puny little minds of cappers - take away from them points AND grant the FC hunter a nice fat bonus (today they are score monks, secluded from the higher ranks of their team score chart).
I would like to see a scoring system along this lines:
Flag capture = 20 points.
Flag retrieval = 10 points.
Flag loss = -5 points.
That way a fast capper would think twice before going in to a second run all alone.
Losing 5 personal points + 10 team points( the retrieving team gain is the attacking team loss) is a great enough of a punishment. Can you imagine the reaction of a team if one of its members lost 30 team points cause he was out for a lone-wolf ego boost?
Losing 10 personal points is discouraging on itself.
There could be different models such as FC=20,FR=20 without flag loss, as long as the main product of the scoring system change is to discourage FC from going at it alone and encouraging team defence.
Also, I suggest that end of the match, instead of the meaningless "the winner is X", the best defenders and capers should be crowned.
Regarding the solution of castrating the playability of the game by means of limiting flag carrier speed: its horrible. I have nothing to add here because those who wrote before me made the case clear (IMHO anyway). But what I would like to do is quote the beginning of one of my favorite short stories by Kurt Vonnegut:
"THE YEAR WAS 2081, and everybody was finally equal. They weren’t only equal before God and the law. They were equal every which way. Nobody was smarter than anybody else. Nobody was better looking than anybody else. Nobody was stronger or quicker than anybody else. All this equality was due to the 211th, 212th, and 213th Amendments to the Constitution, and to the unceasing vigilance of agents of the United States Handicapper General."