A camera’s maximum stills FPS is mainly limited by how quickly it can complete the whole capture cycle for each frame.

On mirrorless cameras, the biggest factor is usually processing speed: how fast the processor can read the sensor data, process it, and prepare for the next shot. That’s why a newer model can shoot faster—the EOS M50 uses a newer DIGIC 8 processor, while the M5 uses DIGIC 7.

On DSLRs, burst rate is limited by both processing speed and mechanical speed. The shutter and mirror have to move for every frame, so their physical cycle time becomes part of the limit. Higher-end DSLRs can shoot faster because they combine very fast processors with more capable shutter/mirror mechanisms. In the case of the 1D X Mark II, dual processors also help support its higher burst rate.

So in short: mirrorless burst speed is mostly processor/sensor throughput, while DSLR burst speed is a mix of processor performance and mechanical shutter/mirror limits.