Investigate why VMs start slower in CI than on workstations

[thrix]

Parent issue: #4491

As noted in #4491 (comment), a VM apparently starts much faster on a regular workstation than in CI.

Task: Investigate why VM startup is slower in CI and whether we can make it:

1. Faster - identify and remove bottlenecks in CI VM startup
2. Deterministic - reduce variance in startup times to make test durations more predictable

Also check whether duration extension is still needed for the provision jobs.