STM32f103 DMA with PWM repeating values
I found a solution while writing this question. I thought I post it anyway since it might help other people. After calling write_all I call wait, which constantly polls the result. This occupied the bus and slowed down the DMA transfer.

Edit: I did not set the right clock multiplier, so the MCU ran at only 8mhz. When running faster, it works great.