When you setup a custom cron, you have one first adittional parameter to specify seconds (not possible in a system crontab).
If you don’t care (like me 
and leave *, you will execute same task every second, and this will hang n8n/host.
It would be nice to put a default 0 on custom Crons to simulate what it happens in a system crontab. And change seconds later if you want.