Pipeline agents sometimes are too slow

We have started using bitbucket pipeline since two weeks ago. The main issue we currently have with pipeline is that sometimes the agents that take the job are too slow!

For example, sometimes, a simple docker image building task would take 30 minutes!! However, if I stop it and re-run it again, it could be completed in less than 5 minutes.

I have read the pipeline limitation document but it does not mention anything about the CPU limitation. I think it would be a good idea to explain these kind of limitations in the https://confluence.atlassian.com/bitbucket/limitations-of-bitbucket-pipelines-827106051.html(limitation document) as well.

We need to have an environment to be consistent from run to run. I'm not sure if other users have experience like us or not.