Just from my experience: it takes a while to meaningfully change avg cadence. It took me 6 months to go from 62 avg to 72 avg, and another 4 to get to the 82-85 range. A 90 avg isn't an overnight thing, and may not be the right goal.

As far as going slower spinning, you still have to put force into the pedals, and not just whip your feet around in circles. Output speed is a factor of force, rpm, and gearing, and if you want to be faster, you balance the three. There are times to mash and times to spin. You just have to figure out which is which.