For monitoring, generally should monitor CPU utilisation and load, not just one or the other. A combination of things, gives a decent idea of what is going on. Memory usage as well.
The first thing I suggested as increase from 2cpu to 4cpu, because you were way below the recommended specifications as per the documentation. The requirements have to be met, else everyone will just reply and say, specs are too low and there won’t be any further help until those have been addressed. There is no need to go higher than this, until your server exceeds the limits as per the matrix: Reference architecture: up to 1,000 users | GitLab
yours is good for 500 users with 4cpu and 8gb ram, but each usage scenario varies. 4cpu at 2GHz, isn’t the same as 4cpu at 4GHz, so in some scenarios, even with 500 users, CPU requirements might be higher than the matrix.
EDIT:
Checked AWS c5.xlarge is 3.6 GHz. So if for example 2GHz cpu’s, then 6cpu or higher may be required, assuming 1.5GHz different.
I run on 4 x 2.4GHz CPU and this is fine for me, no performance/load issues.
Theoretically mine is probably lower than required. If we take 3.6GHz and multiply by 4cpu, then we have approx 14.4GHz total. For 2.4GHz, I would need 6cpu to get to 14.4GHz to make it equivalent. Maybe not a brilliant calculation, probably a bit simplified, but can give an idea for figuring out the requirements perhaps.