BuildStream master seems to be slower than bst-1.2 branch

Summary

Compare:

• https://gitlab.com/BuildStream/buildstream/-/jobs/176370367 (bst-1.2 - commit 38821c72): 464 minutes 10 seconds
• https://gitlab.com/BuildStream/buildstream/-/jobs/176370365 (current master - commit e274adf8): 598 minutes 44 seconds

As you can see they are building exactly the same (freedesktop-sdk-18.08.28-0-g29cf2127554cdd83a1532afa53824b23cee2eed3):

• bst-1.2: https://gitlab.com/BuildStream/buildstream/blob/38821c728cbd0531507cca76aac816b8428129fc/.gitlab-ci.yml
• master: https://gitlab.com/BuildStream/buildstream/blob/e274adf868dc538d6ae678d2f5a9e4d636475c9a/.gitlab-ci.yml

The machine used to build is exactly the same as well (It's the one used for the overnight-tests tag)

What is the expected correct behavior?

master build should be, at least, as fast as the bst-1.2 one)