Hello my friend!
Camunda history cleaning is performed through jobs and batch sizes.
In other words, the cleaning speed will depend on how many jobs you configure to run the cleaning, and the size of the batches that you will execute at a time in each job.
For example, if you set 2 jobs to perform cleaning, and set the batch size to 500, you will have 1000 cleaning executions in parallel.
But you need to be aware that the jobs used to execute the cleaning are the same jobs that the Camunda engine uses to execute time-triggered activities (timer events, activity retries…).
Cleaning speed also depends on your database configuration and resources.
Anyway, I suggest taking a look at this official Camunda documentation, which will help you better understand what I explained above, and also how to configure this.
I hope this helps!
William Robert Alves