Potassium is a mod for Project Arrhythmia that I have been working on for 7 months. The main goal of this mod is to help people with older hardware to play the game as well as extending the possibilities of the Project Arrhythmia editor by allowing creators to use more objects without having to worry about optimization.
GitHub repository: https://github.com/Reimnop/Potassium

Note: Potassium mod is only compatible with Legacy branch. For information on how to switch between branches, click here.
Only proceed with the following steps after making sure you're on Legacy branch.

Step 1 - Download the following files:

• BepInEx modloader (x64)[github.com]
  
• Potassium mod DLL[github.com]

Step 2 - Extract the contents of the downloaded BepInEx ZIP file into your Project Arrhythmia folder (right click on the game in Steam Library, click Properties, click Local files, click Browse).
Your Project Arrhythmia folder should now look like this:

Step 3 - Launch Project Arrhythmia from Steam, wait for the game to load, close it. If you've done everything correctly, inside the BepInEx folder which is in the Project Arrhythmia folder should now have a "plugins" folder:

Step 4 - Copy the downloaded Potassium DLL into the plugins folder:

Step 5 - All that's left to do, is to launch the game and enjoy the boosted FPS!

Potassium aims to boost your FPS by a huge amount, but by how much?
According to my tests, it can boost the FPS by up to 8 to 10 times in best case scenario!
On average, your FPS will be doubled or tripled (not guaranteed), depending on your system and your hardware.
This is thanks to the nature of Potassium - it works by changing how Project Arrhythmia works internally completely. It changes how the game manages objects (how the game spawns objects), how the object's animations are handled and it changes the tweening algorithm in favor of a much more optimized version.
Some screenshots comparing Project Arrhythmia without and with Potassium (with FPS counter):

Level: Peer Gynt V2 by ol666

Level: Nasty Nasty Spell by Cavespider3 + vide0gamer

Outdated showcase video with Potassium bugs that have been fixed in version 1.4:

Even though this mod is finished, due to the nature of its implementation, there might be some minor visual glitches. Since version 1.4, most of the glitches and bugs have been fixed, however, I believe there might be some that I haven't spotted and you are free to give me feedback by leaving comments (I'd appreciate it :D).
Editor is not supported and will automatically fallback to the old engine.
Levels that lag due to gradient spam might not have an FPS boost or have even lower FPS due to the fact that text components are handled by Unity and I can not optimize this.

A massive thank to the enchart, MoNsTeR, JoshyM123 and the Project Arrhythmia Discord for giving me feedback and suggestions for the mod, and a massive thank you to Vitamin Games for creating this awesome game and community!