ASYNC business rule jobs do not process in a timely fashion. This can be because the business rule (with 'when to run' = async) ) is configured with a priority greater than the OOB value of 100 eg 5000, which means these jobs will have a lower priority than the majority of the jobs on the platform, so they will only run once all existing jobs have successfully executed.
1) Identify the Business Rules with high priority value
System Definition > Business Rules - look for an ASYNC BR with a 'priority' higher than 100 (i.e. 5000)
(you may need to customise the list view to include the priority column)
If they exist , the jobs that are spawned by these business rules will be processed after jobs with higher priority (lower number) have been processed. This is expected behaviour.
You can view the list of o/s jobs by opening sys_trigger.list, and then sort by priority.
Ensure that your jobs/BRs are configured with the correct priority for your business needs. Most jobs are configured with the priority of 100, so if you just want these ASYNC business rule jobs to join the 'normal queue', then set with a priority of 100.