Actually, there’s also a lot of things cannot be optimized/changed in the language to keep it compatible with the past code. Actually I feel there are a lot of optimizations in the compiler to make a program run faster or safer, not necessarily faster compilation times. But this irrelevant to me, with modern CPU it’s possible to compile UE4 (nearly 3 million lines of code) in 15 minutes or less. What was most annoying: UHT running on a single thread and often taking a minute to complete its task. Thank God, it’s fixed 
C++ 20 modules are more interesting to me than compilation times 