The git branch source provider inserts a 15 second delay when running a job in a multibranch project in some cases and then inserts another delay of 15 seconds before starting the actual work of running the job.
The GitHub branch source provider does not insert those delays. Users should use the higher level branch source providers (GitHub, Bitbucket, GitLab, Gitea) rather than the branch source provider from the git plugin.
Issue is visible in my jenkins-bugs repository (currently with 170 branches).
Git branch source time stamps
Notice the elapsed time gap between "Branch indexing" and the next statement, then the elapsed time gap between "Seen 170 remote branches" and the next statement.
00:00:00.001 Branch indexing
00:00:16.322 > git rev-parse --is-inside-work-tree # timeout=11
00:00:16.326 Setting origin to https://github.com/MarkEWaite/jenkins-bugs.git
00:00:16.326 > git config remote.origin.url https://github.com/MarkEWaite/jenkins-bugs.git # timeout=11
00:00:16.329 Fetching origin...
00:00:16.330 Fetching upstream changes from origin
00:00:16.330 > git --version # timeout=11
00:00:16.333 > git --version # 'git version 2.11.0'
00:00:16.333 > git config --get remote.origin.url # timeout=11
00:00:16.337 using GIT_ASKPASS to set credentials MarkEWaite github username/password
00:00:16.337 > git fetch --tags --progress -- origin +refs/heads/*:refs/remotes/origin/* # timeout=11
00:00:16.932 Seen 170 remote branches
00:00:31.153 Obtained Jenkinsfile from ccab97146f34497e359bfa594aaefb634f986ae9
00:00:31.153 Running in Durability level: PERFORMANCE_OPTIMIZED
GitHub branch source time stamps
Notice here that the time gap between started and "Running in Durability level" is less than 1 second instead of over 30 seconds.
00:00:00.000 Started by user Mark Waite
00:00:00.093 08:48:10 Connecting to https://api.github.com using MarkEWaite/****** (MarkEWaite github username/password)
00:00:00.650 Obtained Jenkinsfile from d9a27419cf7a554423f5039afdd5b32c95d59c5e
00:00:00.651 Running in Durability level: PERFORMANCE_OPTIMIZED