Upgrading the wrong component produces no improvement and costs the same as upgrading the right one. Identifying the bottleneck takes a few minutes.
How to find it
Run the game with a monitoring overlay showing processor and graphics usage.
If the graphics card sits near a hundred percent and the processor does not, the graphics card is the limit. This is the usual case at higher resolutions.
If the processor is near its limit on one or more cores while the graphics card is not fully used, the processor is the limit. More common at high frame rates and in strategy or simulation games.
If neither is maxed, something else is – often memory or storage.
The usual answers
Graphics card, for most people playing at higher resolutions. The largest single lever for visual settings and frame rate.
Memory, if you have less than sixteen gigabytes. Running out causes stuttering that no other upgrade fixes.
A solid-state drive if you are still on a mechanical one. Does not affect frame rate and transforms loading times.
What often disappoints
A faster processor when the graphics card was the limit. Common and expensive.
A high-end graphics card paired with an old processor, which cannot supply frames fast enough to use it.
The free improvements
Cleaning dust from coolers, which restores performance lost to thermal throttling. Updating graphics drivers. Checking memory is running at its rated speed in the firmware settings, which frequently it is not by default.