I worked in one product that a "simple" re-architecture from rails 3 to rails 5+(async-as-most-as-we-can) was more than enough to handle MAU into millions and simultaneous sessions in high thousands. Enough for a big exit for the founders indeed and having an edge against other players using golang, but with double the headcount than my case.

We also got 98~100 lighthouse by using pure JS with sprinkles of stimulus after the initial rendering.