It's the sad reality of MMO games that there will be bots and farmers. As long as players are willing to pay real money for ingame currency and items, the people willing to take that money will keep trying to corrupt the system. Guild Wars 2 is one of the largest MMOs on the market, so it has to deal with what might be the highest concentration of these people outside World of Warcraft.

To combat this problem, ArenaNet has introduced a series of anti-farming measures into the game that work on the principle of diminishing rewards. The more you complete something within a certain time period, the less reward you get. The intention is that it will punish bots and farmers but not legitimate players who can just move to the next dungeon.  Game Designer Jon Peters had this to say on the official forums:

You should not see any diminished returns unless you.

1) Run the same exact chain over twice in a row

2) Complete multiple dungeons in 30 minutes or less each.

At this time, dungeons should one of the most efficient ways to level and get gold, and it is not our intention to make them feel like a grind. This is a very large game, and we have a lot of people playing it in many different ways. Right now we are working hard to make sure there are not any ways for players to circumvent the normal rewards in the game and then we will be able to look at the actual rewards and make sure they require a reasonable amount of investment.

Many players have taken to the forums and Reddit lately to complain about these measures. While their success against rule-breakers is arguable, it seems that the new code is having a harsher than intended affect on actual players. It seems that in trying to solve the problem, ArenaNet may have gotten a bit over-zealous. Content Designer Robert Hrouda took to the forums again to explain.

The “diminished returns anti-speed clear tech” that went in last patch is NOT functioning as intended. It is currently bugged. We’re working on it, and will have a solution as soon as possible.

At least they know the problem exists and are working on it, which is better then ignoring it or allowing farmers to devastate the ingame economy. It's happened for many other MMO titles and it looks like ArenaNet just went a bit too far this time trying to stop it. Expect to see more updates on this sort of thing in the near future.