I think the following might be of help. I came across it on these forums after I had tried to install Mojave three times with no success and the reason for it might be this file which has followed our upgrades over the years. Mine dated back to 2005. Anyway, I followed the instructions as described and could install Mojave without any problems and my new iMac is now snappier than it was with High Sierra.
A. If you can log in (even if slow), or you're still on High Sierra, and have not yet upgraded, follow these instructions:
1. Start the terminal app from Applications/Utilities
2. Check if the file exists, by typing
ls -l /etc/sysctl.conf
If this returns a result like the following, then the file exists. If it does not, then your issues are caused by something else
-rw-r--r-- 1 root wheel 136 24 Nov 2013 /etc/sysctl.conf
3. Simply rename the file by typing in: sudo mv /etc/sysctl.conf /etc/sysctl.conf.bak
(You’ll be prompted for your password)
4. Restart your computer, and you should be all set, or if you've rolled back to High Sierra, you should be good to attempt the upgrade again.