This "rails/ruby is slow" myth really needs to die. Sure, rails is too slow for certain very large companies at very large scales, and if you're in this position you already know it. However, rails is fast enough for 99.5% of usecases, gitlab included.

As to memory usage, I will concede it is often somewhat high with rails, but there are solutions, such as jemalloc that have helped me in the past.