At what point of optimization does it turn into 'real' high performance programming?

https://en.wikipedia.org/wiki/No_true_Scotsman

>At what point of optimization does it turn into 'real' high performance programming?

somewhat past optimizing the frame count of an entirely empty scene.

on that matter : is it a game engine if there isn't a game?

I totally agree with other comments though -- if there is no pressure to meet specific metrics or accomplish certain things with the product then there is no real pressure to improve past a window or framebuffer drawn to video, just declare it's a game engine that makes a million FPS and throw it on the portfolio.

game engine work gets tough (and rewarding) with 1) goals and 2) constraints -- without those two it's more or less just spherical-cow style work that is too ambiguous or vague for real application.

When the goals are defined. What happens here is you make your cool particle System which is 10x faster than Ue5 but ignore that it uses all the ram or whatever.