Looking for a beginner flight controller board for a fixed wing sailplane

PX4 Autopilot
6

The R15 is based on the slower STM32F4 cheap while the Pixhawk4 mini is based on the faster STM32F7.

If you wanted to go even faster, you could consider the mRo PixRacer Pro which is based on the STM32H7 but it’s considerable more expensive.