Anti-cheat software does real work while you play, so it is fair to ask what it costs in performance. The honest answer is: usually a little, occasionally more, and it depends.
What anti-cheat is doing
While a game runs, its anti-cheat is also running — scanning memory, checking code integrity, watching for injected modules. Some anti-cheats also keep a small component active at system startup, before the game is even open. All of this uses some CPU and memory. It is not free, but for most modern PCs the cost is small.
When the impact is minor
On a reasonably current PC with headroom to spare, anti-cheat overhead is typically minor — a few percent of CPU, a modest amount of memory, rarely something you would notice in normal play. The game itself uses far more resources than its anti-cheat.
When it can matter more
The picture changes at the edges. On an older or low-spec machine with little headroom, anti-cheat overhead is a larger slice of a smaller pie. Boot-time components add a little to startup. And occasionally a specific anti-cheat and a specific system configuration interact badly, producing stutter or conflicts — these are real but are exceptions, not the rule.
How to tell if it is the cause
If you suspect anti-cheat is hurting performance, compare like with like: a game with a heavy anti-cheat against one without, on the same PC. Check whether issues appear only in protected games. Keeping drivers and the anti-cheat itself updated resolves a good share of the genuine conflicts.
The takeaway
For most players on capable hardware, anti-cheat's performance cost is small and not worth worrying about. It matters more on older systems and in the occasional bad interaction. It is a real cost — just usually a modest one.