Make Ubuntu 18.04 lighter?

Refer to this article for making gnome desktop faster. To summarize the six steps in the article:

  1. Disable or Uninstall Extensions
  2. Turn Off Search Sources
  3. Disable File Indexing
  4. Turn Off Animations
  5. Install Lighter Alternative Apps
  6. Limit Startup Applications

The linked article above links to another article with steps you can try:

  1. Show Hidden Startup Applications
  2. Fix Bugs That Slow You Down
  3. Install Adaptive Readahead (Preload) Daemon
  4. Decrease Swappiness
  5. Upgrade Your Hardware

As said in comment you would probably have to choose a lighter desktop environment.
But you can try some tweaks, i have done this and get some improvement

Disable Search and disable windows animations

gsettings set org.gnome.desktop.interface enable-animations 'false'
gsettings set org.gnome.desktop.search-providers disable-external 'true'

OP in a comment stated:

@user535733 gnome-shell uses 15% on both cores at idle, and when clicking on the apps menu, the icons move very stuttery. Although, this is in the live cd. I'l try installing. – Tim Leijten

Performance when running on a live CD is going to be very different from when actually installed, simply because of where the system reads data from. Reading from a CD (or even a live USB) is painfully slow, and it will show in your system. Once you install it to a hard disk (even HDD, not necessarily SSD) you will have a much faster read speed, and the system will feel faster.