An http call to trigger a build using http://host/p4/change will hang when a build is already running.

Expected: returns immediately, build scheduled
Actual: waits until current job finished, build scheduled

This means you should not syncronously call curl within a perforce change-commit trigger as it will delay completion of the submit. Instead, you should asynchronously make the /p4/change call as other network issues, not just this issue, can also delay the response.

Repro:
add a build step to 'sleep 100'. Start a build. Now issue the curl call to /p4/change. You'll see curl doesn't finish until the build finished.