I have created a multibranch pipeline job with this integration plugin, the Bitbucket webhook was created automatically. Now I have the following scenario:

1. I do a git push locally
2. The push arrives to Bitbucket and Bitbucket fires up the Jenkins webhook immediately
3. It takes 1-3 minutes until Jenkins registers the change (with the timestamp of the push from Bitbucket) in "Multibranch Pipelne Events"
4. The "multibranch pipeline scan" gets triggered and schedules a build immediately

Consider the following log line from "Multibranch Pipelne Events":

[Wed Oct 28 13:54:00 CET 2020]

com.atlassian.bitbucket.jenkins.internal.trigger.BitbucketWebhookConsumer$BitbucketSCMHeadEvent UPDATED event from MY_REPO with timestamp Wed Oct 28 13:50:51 CET 2020 processed in 1.1 sec 

Please note the 3+ minutes difference between the git changes timestamp and the timestamp of this log entry. With the 3rd party Bitbucket branch source plugin we get our pipeline scan almost immediately (usually in 1s) started.