Pushing a big gear slowly requires your leg muscles to do all the work. They will tire quickly. Spinning a lower gear faster allows your cardiovascular system to share the load.

Don't try to jump from the 70s to the 90s immediately. Shift down and try to keep it at 80. After a couple of weeks go to 85, then 90. It takes a while to train your legs to go faster. Like you I was probably pedalling in the 70s when I added the cadence kit to my computer. I tried keeping it between 80-85 and was gasping after a half mile. The next day it was a lot easier.

Now I can maintain 100-110 for miles on group rides, occasionally shifting up and doing 95-100 to take a breather while still maintaining the same speed. Another thing about training yourself to spin faster, if you need a sudden acceleration, it is much easier and faster to accelerate quickly by spinning a smaller gear even faster than by trying to slog a big gear faster.