Upgrading hardware is always one of the crucial things that could change your game. Between those 'trinities', you should choose GPU (even CPU if that is possible) upgrade/latest model as it'll smoothly handle frame rates that VR demands. RAM and SSD are also important even though you could upgrade them easily. However, if you have a limited budget and/or low spec on your PC, don't worry because even some VR projects could still achieve and are capable of delivering exceptional quality, even on lower-end hardware, when optimized effectively.

First of all, know that hardware is one part of the solution. Many projects can deliver great quality even on low-end hardware when properly optimized. But let's say your app is optimized. Now you can look at the hardware. With standalone hardware, the only upgrade is to buy a newer device (eg. Quest 2 -> Quest 3) If you are on PC, things are a bit more complicated. Here, you need to check where your issue is. Without going too technical, we will use Task Manager. Is your GPU always at 100% when running the app, a GPU upgrade might help. Same for the RAM. CPU is more tricky, as most apps use 1 primary core. So switch the manager to per-core graphs, and check if 1 core is peaking at 100%. If so, you might need a CPU upgrade.

Assuming the undesirable performance is not a function of a poorly optimized application, or the limitations of the headset itself, you can reasonably look to your PC hardware for potential upgrade. While running your app, Task Manager provides helpful insight on GPU, Ram, and CPU usage. 100%, or a disproportionately high usage of any of the above would be reason to investigate further. In-game performance analytics, or tools like CPU-Z / GPU-Z will provide a level of deeper insight on how each individual component is performing. As a best practice baseline, I'd suggest 16gb RAM and a Solid-State Drive. Even better would be an NVMe SSD.