Last time i checked c++ debugger still used 16 bit line numbers. My attempts to make a plugin for vs also left an impression that it contains large amount of modules glued together using various technologies. Things like 5 layers of c# API wrappers named api10, api20, api30 on top of some c code communicating using COM objects. I wouldn't be surprised if they still had some third party libraries for which they only have a binaries and vendor doesn't exist anymore.

Compilers and IDEs are resource intensive pieces of software and VS is over twenty years old. I can imagine innumerable opportunities over the years to increase performance by assuming the size of certain variables in memory. That kind of stuff would have been a big deal when compiling C++ on a 333mhz processor.