MR Pipeline toke more than 50+ seconds to be triggered

## Issue

### Context
- **Gitlab plugin version**: 1.5.20
- **Gitlab version**: 13.6.0
- **Jenkins version**: 2.277.3
- **Job type**: Pipeline

### Logs & Traces

### Problem description

When opened a new MR from Gitlab UI, Jenkins pipeline is instantly triggered.

However, when update the MR with a simple new commit, pipeline took 50+ seconds to be triggered.

Different between two webbook Request body is action = open for new MR and action = update for new commit.

At the same time, close then reopen the MR also take 50+ seconds for pipeline to be triggered.

Here is gitlab webhook setting page screenshot