It's not about a single 50mil element array. It's about sub-optimal code running in a bunch of places and it adds up. But in any case this probably won't be the bottleneck.

Still I am a fan of running the most optimal code possible on the server. Absolutely no reason not to.

Client-side js is different. Often times algorithmic optimizations have no impact (unless we are talking about animation.)

I would not trust people who do not respect optimizations like these to run code on my server.