Urbackup Docker Compose CPU Limit does not works


I have Urbackup running in the Docker container on a Qnap. When more clients are doing backups, the CPU goes to almost 100% (Qnap). Now I wanted to limit the RAM and CPU usage in the Docker Compose YML file, but that doesn’t work.

I use the compose file version 2 and have entered the following.

version: '2'
    image: uroni/urbackup-server:latest
    mem_limit: 1024M
    mem_reservation: 512M
    cpus: 0.50
    container_name: urbackup
    restart: always

The limitation of the main memory works, but the CPU utilization is then displayed at “docker stats” about 300%. What am I doing wrong? Is my syntax wrong?