Some games (not all) including copyright systems like Denuvo (DRM) checks or Redshell in order to spy & log the gamers activity which has an negative impact of the game performance itself (longer loading times, FPS drops or even crashes). A filter-list to prevent such stuff can be found here.
Keep in mind that telemetry has (in normal circumstances) no effect (or less) on the gaming performance (if it was proper implemented) and you should consider to leave it enabled in order to help the game developers (e.g. for crash reports or to improve the anti-cheat mechanism) however, such systems scanning your entire OS and this might invade your privacy. I highly recommend that you read the terms of service before you opt-in.
The biggest impact on the game performance itself is done by the copyright-protection system and its implementation and can have a negative impact which results in longer loading times or small FPS drops. The rumour that Denuvo cripples your performance is not entirely true, it's depending on several factors in most cases you lose around 2-3 FPS (which is considerable not "crippled" - depending how you see it).
- Use GOG releases because they are DRM free, some Epic Game Store and Steam Store Games are also DRM free but the Client itself is not.
- Play cracked versions (possible malware risk) or emulate them in a VM/Wine/Sandbbox. You still can support the game developer by buying the game, just don't install it via official ways.
- Check if there are confirmed and clean cracks from trusted groups which avoids losing some performance via VM/Sandbox (as mentioned above) but the risk is still there (!) that you get infected (via re-packs, drive-by, Malvertising etc). It is up to you.
- Check if you need the Game Client and read the privacy policy before you install it on your OS!
- Manually removing the analytics code requires to manipulate the game client and it's files and is dangerous because that might result in a ban, the game client should provide an opt-out toggle for such tracking. A better method is to block the request with a firewall/hosts file.
- Depending how well it was implemented (example), telemetry (the running processes) never uses much resources (depending how you define "much").
- Telemetry can run in the background or even without user consent, the best scenario would be if you can manually control it opt-in/opt-out and submit only telemetry when you want it
- Checks could be integrated to scan the current running applications and if they are currently in fullscreen or not (e.g games)
- The Telemetry and his task schedule can be configured to run only on specific circumstances e.g. if the PC is idle
- Depending on how big the submitted data are or how small they are you will never notice it (not in-game and not while you use other applications)