I'd love to have a tachometer. I usually use the "Seat of the pants" measurement.
My 15-90 started slow. I thought that it had to do with the fact that the grease ports weren't doing their jobs. When I got it, there was a crust of grease on the top, and the bottom of the wick (closest to the motor) was white like it was fresh from the factory. I gave it a tiny little dab of grease right where the grease wicks were supposed to supply it, filled the grease tubes per Dave McCallum's "5 second squeeze" suggestion, and started it up. It was stinky and slow then it started speeding up, it was really neat to see. Info is good. We can make "informed" decisions with it.