i could not achieve the vermagic number as i did years ago so suspect this to be the cause
git clone https://git.openwrt.org/openwrt/openwrt.git && cd openwrt
git checkout v19.07.8
./scripts/feeds update -a
./scripts/feeds install -a
It is best to move on to a supported version of OpenWrt. This has been EOL and unsupported for roughly half a decade and has accumulated a large number of vulnerabilities.
Is there a reason you're not using a recent version of OpenWrt?
my priority is performance
What hardware are you running?
I get that, but you should also consider that 19.07 isn't suitable for use anymore due to all of the security vulnerabilities that have been discovered in the past ~5 years (that will never be patched).
solution is to skip make kernel_menuconfig