The CPU share setting is a percentage relative to each app. So, you should make tinytinyrss slider to be 20%. That way, it will take less CPU compared to other apps (which are set to default to 80%).
There's a more detailed explanation here - https://www.batey.info/cgroup-cpu-shares-for-docker.html . When 50%, we set the CPU shares value to be 50% of 1024, when 20%, we set the CPU shares to 20% of 1024 and so on.