In the last few releases the compilation time of qtwebengine has increased a lot.
qtwebengine 5.9 - 5.14 were 2-3 hours on my system (Intel(R) Core(TM) i7-7600U), qtwebengine 5.15 is around 9 hours.
Also, if I remember correctly, the compile targets increased from 12000 up to 20000.
It would be nice, if you can provide a precompiled version for low hardware systems.
*** This bug has been marked as a duplicate of bug 669082 ***
Please do note that USE="jumbo-build" was disabled for the time being recently, due to issues some people have had with it, and that might explain the big jump you're seeing.