Hi! I found this thread after we chatted via Support. I'll go ahead and leave some of the tips we discussed here in case it should help other users:
Automations stuck in pending status are typically a result of overall base performance/load. If the trigger is "When a record is updated," for example, and you're using the API to programmatically update records at a high frequency, it could overwhelm the base and cause many pending runs. As automations run in a queue, if one automation run gets stuck or takes longer to complete, the other runs that are queued up above it will have to wait until the current run is complete.
Furthermore, when using the Gmail: Send email action, Airtable must respect rate limiting which Gmail imposes, so we're unfortunately not able to manually reprioritize the pending runs on our end.
Note that turning off an automation will cancel the pending runs that are queued. If you decide to do so, you will need to return to the canceled runs and either run them manually or devise a one-time automation to trigger the automation to run for the relevant records in smaller batches (perhaps by leveraging a Checkbox field).