Hello,
It should not be too loose or too tight

It is more a question of your leg weight. With heel-up you can go only up to moderate tempos (probably 16s at 160 bpm).
When you go faster than that, you have to switch to heel-down (or something in between), otherwise muscle tension kills your legs too fast.
You CAN play that fast heels up, but for a bar or two. If it is not the case, you will have to relax your feet more when you go faster, which means that your heels will go down... or way down (same as with hands - the faster, the more relaxed).
At fast tempos you have to rely more on the momentum control than on forsing the beaters (again, like with hands). I would recomment to try different positions of your toes along the pedal boards - this creates different momentum, so you may find it easier to play at fast tempos when e. g. your toes are right in the middle of boards (not close to the chains).
All of the above is my personal view of the problem and can't be treated as an absolute truth

Hope it helps.