If you put your system under load, it needs more power, if you only have a weak power supply, this can definitely become a system brake.
A power supply works with the highest efficiency if it has a load of between 50 and 70%, how efficiently the NT works can be recognized by the classification (bronze, silver, gold, platinum etc.). The higher the rating, the more efficiently the system works and enables a higher yield of effective performance.
With 500W power of your NT you can simply deduct 10% and consider it a power loss, so you have a maximum power of 450W with maximum load. The higher the load, however, the more heat is generated and that's what you don't want in any PC.
If you have poor cooling performance on top (a midi tower is only suitable for a gaming system, for example), your performance will also be reduced because the hardware starts to clock itself down to protect against damage and thus keep the temperature down,
Before you take any money and work into your hand to replace the power supply, you should take a look at your system and the software. If you have a fairly old system and a lot of garbage on it, you may well be slowed down by some software or by garbage. Viruses, Trojans and Co. Brake properly, you should never underestimate that.
Bad software (usually any freeware) can also slow the system down, especially free anti-virus software like "Avira" or "Avast".
The easiest thing you can do for checking is to simply rebuild your system, update all drivers and then set the software correctly. If you do not know what to set, you should keep your hands off and drive with the "standard system".
If that still doesn't help, you can consider installing a new NT, but you should go to min. Use 750W, ideally with a gold certificate or higher. You don't actually need platinum, so gold would be enough.
But I don't want to give you a guarantee that the power supply will fix all problems, that is quite possible, but it can also simply be due to the software.
The rest of the hardware should at least reach more than 120 FPS at 'Low', it is important to note that you have not limited your FPS externally. This can also be set in the system settings of your GraKa.
