Audi,
Like I said, "the devil is in the details".
You will practice according to how much detail you wish to incorporate into your form.
Do you want to train ALL the details precisely, or is it enough to get it "close"?
Everything I've ever understood about form time is that the slower the better, for many reasons.
Not the least of which is for detail oriented training.
If you slow it down, you can train every single little nit-picky detail, one by one. When you speed it up after many, many practices those details will smooth together into a cohesive, complete form that will flow together smoothly and without breaks in the flow.
Martially this means that you've got a lot of "hidden" things going on at speed. They are not apparent to others, so you're getting the benefit of "hidden" movement against your opponent. One of the mainstays of TCC martial theory is to "hide" your intentions and your movement as much as possible from your opponent. If you can turn your ankle, shift your weight and issue and no one sees or knows you'ver turned your ankle or shifted your weight, then you've successfully hidden the soon to be coming issuing against your opponent.
Does this make sense?
So if you're doing your practice long forms in ten minutes, you're cutting off one of the most beneficial part of your form training.
Slow it down, slow it down more. Make each and every move distinct, meaningful, complete in and of itself. Thread them together, but keep them clearly and distinctly seperate.
At "martial speed" these small movements and shifts that are so greatly exagerated in slow form practice will naturally follow you. You will "meld" them together effortlessly into something that appears to have none of the desired qualities, but still maintains each one in it's own right.
In this way, your opponents will never know what you're going to do next, but whatever it is it will still maintian the principals and therefore be infinitely more effective.