As i can understand - the main process in Arduino is digging lots of files of the source codes that refer to libraries files that refer to the SDK files that compile and link all these tons of files. And B4R is wrapping over all this in addition.
So, speed trouble is just the OS file system.
But if SSD is used for sure.... no another way excepting the RAM-drive to try.
p.s. my opinion now is that Arduino 2.x work with B4R 4.x is around the same slow like Arduino 1.8x was before. Maybe just a bit slower.
First compilation is always maximally long, next pre-compiled files are reused, so only much changed modules need the project to be re-compiled again.