If jobs in your pipeline are queued in usage queue and want to know which jobs are blocking your build, you can use the following API endpoint.
vcsis the name of the VCS, either
orgnameis the name of the organization or username
projectis the name of your repository
CIRCLE_BUILD_NUMthe built-in environment variable associated with the specific build in question, which can be found in the UI
You can find the build URL with "build_url" key from the response.
For those on the Performance Plan getting stuck in the Usage Queue, you can request a concurrency increase by writing to firstname.lastname@example.org.
Please note that this is an undocumented API endpoint and might removed or modified without prior notice.