I am also a fan of esp8266. Maybe clock-for-clock, the stm32 has the edge, but the esp has loads of ram and a user-configurable clock speed of 80 or 160MHz. And at around the same price as most stm32 development boards, has built-in WiFi. Adding WiFi to other dev boards costs around the same as buying an esp dev board.
My favourite esp dev board is the Wemos D1 mini. $4+shipping.
To demonstrate it's speed, in this video it is calculating Conway's game of life on a 512x512 grid at several tens of generations per second, and updating a 128x64 oled screen at the same time.
Andreas does some (not very scientific) tests in this video to compare the speed of the Maple Mini to an ESP and also the Arduino Due and the Pro Mini.