It is not necessary to import time varying velocity to speedup an LES simulation. In fact, your precursor simulation can even just be RANS and doesn't have to be LES. Here are some LES-specific solution strategies
The only addendum is between step-1 and step-2 for your particular case - here, write-out the profile file for three velocity profiles at the inlet. Import this profile file into the u-bend case and attach it to the inlet and then start the LES run.
With respect to visualizing the instantaneous contours - the variables that are available by default are in fact the instantaneous variables.