[GitLab] Issue creation modal takes too long to load

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

1. Have a GitLab org with many projects
2. Integrate it into Sentry through the GitLab integration
3. Link Sentry projects to GitLab repositories
4. Pick an issue in Sentry and try to create/link it to a GitLab issue

### Expected Result

The modal loads immediately.

### Actual Result

The modal takes 19s (or more) to load.

- The customer has checked the logs for Chrome and the DNS AdBlock - neither are slowing down or denying access to any requests.
- In incognito mode with no browser extensions it is exactly the same.
- The network is 1GBps, with 6ms latency, tested on their machine using fast.com
- No VPN
- Nothing in any firewall rules that could be causing the response time from Sentry's end to be slow

Org and video repro in [the shadow ticket](https://getsentry.atlassian.net/browse/FEEDBACK-2402?atlOrigin=eyJpIjoiZWFkMzliY2EzOThhNGMyMGE3NGUyZTI3NTg5MzFmYjYiLCJwIjoiaiJ9) as I couldn't reproduce the issue on my end when trying to create a GitLab issue in the customer's org.

### Product Area

Settings - Integrations

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Start with the GitLab integration issue-creation modal and follow the reproduction steps using an organization with many projects. Review the linked shadow ticket FEEDBACK-2402 for the organization and video reproduction, then compare the modal's load time with the expected immediate result. Done means the modal no longer takes about 19 seconds or more to load.

Written by the indexing model from the issue text.