systemd is what's known as an 'init system'. If your Linux machine uses systemd, when you start the machine the first process to run is systemd itself. Every other process is managed by systemd. It is responsible for managing the system and services that run on your machine, however this is just the tip of the systemd iceberg.
While it's a very powerful tool, it's also a bit troublesome. Linux is based on the Unix operating system and tries to adhere to what's known as the 'Unix Philosophy'. An informal part of that philosophy is 'Do One Thing And Do it Well' (DOTADIW); a piece of software should always have one goal in mind. If you need something else to be done, don't add it to an already existing piece of software, start a new piece of software and make sure they are well designed.
systemd doesn't DOTADIW; it manages (to some extent) power, mount points, network configuration and many other things besides. It's difficult to extrapolate what you need or to troubleshoot issues if it all feeds back to the one, bloated beast.
It also has several issues, and as you might have guessed; if you have an issue with the software that fundamentally runs your machine then you've got a BIG issue because it will affect everything else. Most of the issues I've heard are usually slower performance or troubleshooting issues.
Want a change? There are plenty of other init systems you can try. Why not have a look at Void Linux which comes with runit by default. Or have a look at some of the other alternatives available, this list is up to date and easy to read, with a few quick pros and cons for you to review.
To fix your issue, you might need to have a look at what is causing your GPU to run slow. How do you know that systemd is the problem? It could be the drivers for your GPU, or perhaps the modules weren't being loaded?