The only attention this problem has currently received is in this answerhub post.
The situation is that the speed of C++ compile times have worsened significantly in the latest version (4.17) of UE4. Personally, in my projects the compile times have gone from 3-5 seconds for small changes (in 4.16) to 13-15 seconds (in 4.17). As the answerhub post points out, the compile times go right back to being fast again when the UE4 Editor is closed, which indicates this might have something to do with the speed of hot reloading in 4.17.
I hope this issue is resolved soon, as it significantly decreases productivity while programming in C++.