GitLab ee omnibus is very slow

Gitlab version : GitLab Enterprise Edition 10.7.2-ee
Env : 15GB memory, CPU 2,3Ghz 4core.

Our Gitlab instance is getting very slow. Web-ui take nearly 10 seconds to load each page. I saw systematically about 10 second on TTFB.
I’ve raised unicorn process to 8 without significant improve.
Is there something else I can do ?

Best regards,

Find out whether your bottleneck is CPU/RAM/disk/network/…

Without any knowledge of how many projects your gitlab servers houses, how large they are and the count and nature of CI jobs (and probably a lot of other things) it’s impossible to give advice that is not just blind guessing.

There are about 10 or 12 project on our instance and 2 or 3 CI.
Even when there are no jobs running UI is still very slow.
No network problem or disk.
CPU is only at 10% occupied and there’s 10GB RAM left.
I saw that everytime assets are requesting it takes nearly 10s to deliver.