Thank you for the respons. This could be the reason. My iMac is 4 core with hyperthreading, makes 8 cores.
The manual: "The number of available Compressor instances is determined by your computer’s cores and memory. After meeting the minimum system requirement (four cores and 2 GB of memory), you can add one additional instance for every additional four cores and 2 GB of memory."
So my M1 pro gets the same additional instance as the iMac, both get one. And you have 3.
I've tested both systems with a 20 sec. mp4 from a small camera. The M1 is about twice as fast encoding HEVC but to my surprise H264 on iMac is a little faster. Maybe because the M1 produced a movie with a bitrate of 70.000 kbps although both where set on a fixed bitrate of 50.000 kbps.
The instances doesn't seem to make any difference for single file, it does make a difference selecting multiple files to a batch. In my small test about 20% faster.
As an amateur I don't need the speed anyway but I'm glad that your mentioning of the hyperthreading solved the problem and let me sleep in peace tonight!