Hello Ido,
A scenario can run at maximum 50 runs asynchronous.
Every time you clear 1 run, a next one will take is place. That slow down the process.
If you have too many runs a the same time from different scenarios in your organization , it consumes Make slots which is limited.
By the way if you are a heavy Make user , you might be interested by this Make dashboard to monitor consumption / errors in your organizatoin