Hats off to anyone who can beat 25 minutes on a medium gear. Like you, I can't get the power down - I can spin at 150rpm, but that's just keeping up with the pedals, not propelling the bike.
The 90" gives 85rpm on the flat at 23mph, which is a reasonable cadence for a TT, but it's really needed because the first half mile, where a PB can be won or lost, is slightly downhill and 30mph is normal. In fact, a few years ago in an open TT I did 30mph all the way to the turn with a stiff tailwind directly down the course (an old Roman road). 15mph all the way back though.