We are running a big server (12 threads, 6 cores, 64Gb ram, 2 SDDs raid-0) for our rails app deployed with nginx/passenger.

Unfortunately, pages are taken forever to load something like between 10 and 40 seconds. However, the server is under a really light load, with a load average of `0.61 0.56 0.53`. We have ram used weirdly, `free -ml` reporting 57Gb (of 64Gb) usage whereas `htop` reporting only 4Gb (of 64Gb).

We have checked our production log, and rails request takes something like 100/200ms to be completed, so almost nothing. 

How can we identify the bottleneck?