Why FPS lies and frame-time rules
Average FPS is a one-second average, so it hides short spikes. If most frames land at about 7 ms but one occasionally stretches to 25 ms, the counter still reads a comfortable ~120 while it feels like micro-stutter, because your eye catches the long frame. That uneven gap between frames (frame-time) is what causes the classic 'high FPS but it feels choppy' problem, and no GPU fixes it when the cause is somewhere else.
What actually steals smoothness
Most of the time the culprit isn't the game, it's the background: overlays, launchers, an antivirus mid-scan, cloud sync, dozens of startup apps. Add RAM running at default JEDEC timings instead of its XMP/EXPO profile, and your memory delivers less than you paid for, which hurts 1% Lows in CPU-bound titles. Individually these are small; together they add up to a noticeable loss in frame consistency.
How to see it instead of guessing
Don't trust your gut, measure it. In your overlay enable not just FPS but frame-time and the 1% / 0.1% Low figures (MSI Afterburner, CapFrameX, and the driver's built-in overlay all do this). Run the same scene before and after a change: if average FPS barely moves but 1% Low climbs and the frame-time graph flattens, you've won smoothness, and that often matters more than a couple of extra average frames.
What to tune first
Start with the cheap, safe wins: trim startup apps and background overlays, install clean drivers, enable your RAM's XMP/EXPO profile in BIOS, and in-game cap your frame rate slightly below your refresh with G-Sync/FreeSync active, which often visibly evens out frame-time. Every step has a cost: XMP may need stability testing, a frame cap lowers peak FPS, and disabling overlays loses handy features. So change one thing at a time and verify stability rather than doing it all at once, and if you'd rather not poke around in BIOS, that part is fine to hand off to someone who knows it.
Verdict
A top-tier PC raises the ceiling but doesn't tune itself, and without clean frame-time, a quiet background and a working memory profile even a 4090 routinely gives back less than it can. Watch 1% Low and frame-time, not just average FPS, and make changes one at a time. Sometimes twenty minutes of tuning buys more felt smoothness than a costly upgrade.