Most 32-bit Flight Controllers have a ST Microelectronics CPU (Technically it's a Microcontroller, not a CPU). There are four CPU Families that are used in Flight Controllers: STM32F1, STM32F3, STM32F4 and STM32F7. They're simply known as F1, F3, F4 and F7.
Each CPU family have few different specific CPU models.
This property can have the following values:
STM32F1 CPU is a 72MHz and 128KB Flash Memory 32-bit CPU. It have Two UARTs and lacks a mathematic auxiliary unit. It was widely used on early 32-bit Flight Controllers like CC3D and Naze32.
STM32F3 CPU is a 72MHz and 256KB flash memory 32-bit CPU. It have Three UARTs and do have a mathematic auxiliary unit, witch increase A LOT it's processing power compared to it's F1 predecessor. It is widely used until today, but it's being replaced by faster CPUs.
STM32F4 CPU is a 168MHz 32-bit CPU. It have Five UARTs and 1MB of Flash Memory. It's faster than it's predecessors, but it cannot do serial inversion by software like F1, F3 and F7, so flight controller must have specific inputs for SBUS and SmartPort Telemetry. It is widely today as it's the standard, but it's being replaced by F7 CPUs.
STM32F7 CPU is a 216MHz 32-bit CPU. It have eight UARTs and 1MB of Flash Memory. It's faster than it's predecessors and can use virtually all features of any flight controller software. It is relativelly new on flight controller scene, but some great flight controllers are already using it.