The trainer is to help you develop a tactical eye, not improve your thinking; so faster is better.
Change on tactics trainer

The average time on each problem is based on the average time it takes to solve the problem for each player. Since the score is changed, the average time will change too. A lot of people will disagree with the statement 'faster is better', and I do too. So that's why I wonder: is the change intentional or accidental? And if it's intentional, why didn't anyone anounce that there is a change? It should've been announced at least, because it'll have a big impact on tactics trainer. If it's accidental, I hope someone from the staff will take a look at this.
I've experienced a change in tactics trainer. It seems when you solve a problem at half of the given time, you get a score of +/- 70% instead of 80%, as it used to be. Was this change intentional? Why did you change it anyway? It now encourages the players to move a lot faster as it's much harder to keep your rating even.