Mojave general slowness, network & bluetooth issues

ALL CREDIT TO @Quirkz:

Hey there, may have a solution for you and the others suffering this:

If you're also seeing symptoms like networking failing, bluetooth devices not connecting, etc, and you have been using the mac for a while (or restored it from another machine using migration or time machine), then it's due to an old, out of date system configuration file that is contains settings that are too low for modern macs: '/etc/sysctl.conf'
This file doesn't even exists on new macs, so if you have it, it will cause problems

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. Remove the file: (actually this command will simply rename it, so you can restore it if you really want to mess up your computer again.)

You'll be prompted for your password.

sudo mv /etc/sysctl.conf /etc/sysctl.conf.bak

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.

B. IF you are unable to successfully log in at all (usually those who are running encrypted disks), then you're going to have to do a couple of steps first.

1. Reboot in to 'recovery mode' by holding cmd-R when you power on the computer until you see the apple logo.

2. You'll be presented by a menu with the 4 recovery options. One of them is 'disk utility'. Click on it to load disk utility.
3. Disk utility displays a list of the drives/partitions on your computer. One of these will be called something like 'Macintosh HD'. Select it. Next, On the top right hand corner of the disk utility window is a series of icons, one of which is called 'Mount'. Click on this. If you disk is encrypted, it may ask you to type in your password to unlock it. Do so.

4. Quit the disk utility app. This should take you back to the recovery menu.
5. Launch the Terminal app as follows: In the top main menu bar is a menu called 'Utilities'. Open this menu, and you should see 'terminal'. Click on it to launch.
6. From terminal, rename the troublesome file:
mv /etc/sysctl.conf /etc/sysctl.conf.bak

7. Exit terminal, and restart your mac. You should be able to log in successfully.