Yes I found a solution in my answer here:

https://serverfault.com/questions/1002021/nginx-fails-on-high-load-with-debian10-and-not-with-debian9

accept_mutex changed from on to off in the default value. Setting it back to "on" and Nginx is happily running with 10k requests per second again. I guess it is a combination of multi_accept and accept_mutex which caused my troubles.

regards
Janning