This probably isn't a new forum thread, but since it's here I'll just say I agree that how the timer is implemented should be modified.
1) The "average time" should only be calculated based on people who passed the problem. If they failed, their time should not be a part of the average. This may already be the case, but if it's not, it should be.
2) High times relative to the average should lower the amount of rating gained from a successful pass to a minimum of zero rating gain. High time alone, however, should not result in rating loss. Actively penalizing rating based on time puts too much of a premium on speed (measured in seconds) which in serious chess is generally not as important as actually choosing the best move.
I'm a regular trainer user (3 a day): 10 hours of training time, 650 problems attempted, and I have a pass rate of an even %60. But my rating hovers between 1150 and 1250 because when I start getting into harder problems I start getting nailed on time. I would like to train against harder problems, but I refuse to make myself rush, because I never force myself to rush when I play an actual chess game.
People who pass quickly should be rewarded more than people who pass slowly. But people who pass shouldn't be penalized for high times.
Once I was pretty beast at Tactics Trainer compared to my other chess.com ratings. (It kinda balanced out to the 1200 level.) Then I participated in a bunch of state and local tournaments with G/45, G/60, and G/90 clock setups. Used to the past-paced rhythm of Tactics Trainer, I was pretty bad, let me tell you. Then I got used to the slow, easygoing clocks and now I'm sucking at Tactics Trainer.
Then I discovered chesstempo.com. (Not advertising, just comparing it to chess.com's Tactics Trainer.) ChessTempo is similar to chess.com's Tactics Trainer, but the problems don't have a timer that used to rush me. Used to the clocks I am pretty good at these tactics, but now looking back to chess.com I think, "Why does that timer rush me so much?"
This is why I move to remove the timer of chess.com's Tactics Trainer. Since using ChessTempo I've been experiencing the easygoing feeling at the state tournaments doing their tactics problems, and by removing chess.com's timer, it would be similar to ChessTempo. Plus, right now, the Tactics Trainer is similar to ChessTempo's Blitz tactics program since it has a timer. Comment on this if you agree.