Tachometer for my bike

Is it not [power] cheaper to calculate the speed just every few seconds than adding more logic into each wheel turn?

Absolutely, but you can go for a middle ground: you just store the times away and keep doing what you do every 30..60..90 seconds. This way you do insignificantly prolongate the power-on state per wheel rotation, but have good data to calculate / prep/ update (prep an update) for a graph in desired intervals and may be show it graph on a nokia 5110 (or better) on demand... managing the back lighting accordingly... Don't know the e-paper power consummation on update... (see e-paper GDE021A1 display).