Yep, it uses the source Null used.
Nothing stops them from travelling at the same speed as their casual attacks. Unless someone wants to tell me that Saiyan Saga Piccolo can't move at Relativistic speeds or that Goku can't travel at MFTL+ speeds despite fighting against Beerus who can do just that and both having their Energy Ball feat scale to them because they already downscale from a serious Whis.
Technically the fighting speed should be higher since the energy used for speed won't be waisted on distance, but will be used on speed [or short burst] thus technically making it faster.
But it doesn't matter, being it at least equal to travel speed would put them at infinite speed/mtfl+ [if the infinite size thread even passes, which...I severely doubt it]
Heck, even when the whole 3/4 speed of Whis (Whis doing 487 quadrillion c) thing was going on, assuming Beerus was going half as fast as that, Goku would undeniably be above 182 quadrillion C, after which all other transformations would effectively surpass it by massive leaps and bounds).