Summary: rebuilds in ubuntu in a vm are significantly faster than fedora natively.

"native" is a fedora 29 desktop.  nvme ssd (samsung evo 960) i5-8400, 32GB ram
"vm" is a ubuntu 18.10 vm running in "boxes" on the same machine.

Openwrt git checkout revision: cdb58b2bfe
Procedure is:
git clone
./scripts/feeds update -a
./scripts/feeds install -a
cat > .config
CONFIG_TARGET_ar71xx=y
CONFIG_TARGET_ar71xx_generic=y
CONFIG_TARGET_ar71xx_generic_DEVICE_rme-eg200=y
CONFIG_DEVEL=y
CONFIG_BUILD_LOG=y
CONFIG_DOWNLOAD_FOLDER="/home/karlp/src/dl"

make defconfig
make once to get everything in place.
repeated calls to "time make -j10"
native on the left, vm on the right.

real    1m33.273s               real    0m39.463s
user    2m18.183s               user    0m51.127s
sys     0m41.785s               sys     0m11.110s
                    
real    1m30.110s               real    0m40.412s
user    2m16.333s               user    0m51.652s
sys     0m40.599s               sys     0m11.834s
                    
real    1m32.126s               real    0m39.075s
user    2m17.597s               user    0m50.629s
sys     0m41.156s               sys     0m11.092s
                    
real    1m30.358s               real    0m39.401s
user    2m16.380s               user    0m51.449s
sys     0m40.314s               sys     0m10.662s

• native.html
• vm.html