How to deal with long pre-bled brake line?
Shortening the hoses isn't much more than a 10 minute job on most models. You'll need either a bike shop, or some nice cutters and a new olive and insert which will cost a dollar/pound or two.

Although, as these are moving to a new bike, I'd leave them be and do a more thorough job of looping them until you can measure them on the new bike. What are you using, my first stop would be zip ties and if that slips still, duct/duck tape. When you move them, a little alcohol will lift any sticky mess that's left.

You wont hurt the performance of the brakes unless you kink the hose. Don't do that. They'll want replacing if you do.