Temporal Curtain
The only other class with a comparable issue (Guardians) has a very smart solution to this:
- If you have no swiftness, gain 4s of it.
- If you already have swiftness, gain +1s of it.
We’d basically need the same second effect for TC.
Back when the game was released, running through the line multiple times would stack the duration. As you pointed out, that was ridiculous.
They decided to fix it, and for over a year if someone ran through the line with no speed buff, it would buff them but no other speed buffs would increase the duration…painful to groups. You really hoped that all mesmers running with you knew not to ever use there speed buff if you were in a group.
Currently it will not increase an active duration, but it can add a speed buff if none exists and that buff can have its duration increased by any buff other than the line. While not ideal, it is much better then the first two renditions. I find myself dropping it in group runs just in case someone missed the guardian circles that I am following etc etc.
Temporal curtain is not too bad as is. The only real gripe I have with it is when you cross the curtain with a few seconds of swiftness remaining you get nothing. In particular, using TC with signet of inspiration equipped is risky because you may randomly get 3s of swiftness just before you cross the curtain. Also well-meaning players can give you small amounts of swiftness and thus negate the effects of TC.
It would be better if it simply increased your current swiftness up to a maximum of 12s (+boon duration bonuses). Unfortunately given past experience, if Anet made this change it would probably add bugs to some unrelated phantasm.