I have no experience with TeX ./. LaTeX, but one idea would be that LaTeX is simply not seen as performance-critical – whether your huge report takes two second

I have no experience with TeX ./. LaTeX, but one idea would be that LaTeX is simply not seen as performance-critical – whether your huge report takes two seconds or thirty seconds to compile is rather irrelevant if you do that once or twice a day. TeX being some sort of subset of LaTeX might simply make it faster due to a lack of features. On the other hand, people spend ages trying to optimise HTML.