B4r compilation calls Arduino ide. Does it not use multithreading? Compiling an esp32 project using b4r takes much more time than Arduino ide.
Compared with Arduino ide or vs + visualmicro, the compilation time needs to be increased a lot.
There is nothing that can be multithreaded. The compilation is done by Arduino IDE command line tool.
Make sure to compare the speed after you compile the app once as B4R will keep previously compiled objects.