How close are your GPUs one to another (phisically) ? What are your temperature settings ? It could be that your cards are thermal throttling.
There's a GPU each 9cm along the rack.
When this issue occured just now I turned the rig off, cooled down the whole room, GPU temp dropped by 10C, started the rig and it... launched with the full speed yay!
So your guess may be the answer to my question! I will speed up the fans and see if that solves the problem as actually they spin very slow. Maybe I have to relax my low noise requirements...

Thanks