I don't know what about LaTeX makes it slower than TeX to compile. I always use LaTeX rather than TeX so I'm used to the former's speed.
So recently when I compiled a huge TeX document, I assumed the compile had failed because it returned so quickly -- it seemed to return immediately -- but it completed successfully.
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.
So recently when I compiled a huge TeX document, I assumed the compile had failed because it returned so quickly -- it seemed to return immediately -- but it completed successfully.