Here is an appropriate extract from my personal notes about ESET v. 6 and this topic:
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Using Gamer Mode **************** ESET usually consumes very little ressources compared to other security suites. The default setting for the Gamer Mode is set that it enables as soon as a game or application goes in Fullscreen. This will temporary disable scheduled task and pop-up windows. This is enough most of the time for "normal" games that do not require extensive ressources. If you play a heavy ressource demanding game with advanced grafics and memory requirements, then take these additional steps (from ESET Knowlegebase): a) Scan only executable files By default, your ESET security product scans a file whenever it is created or accessed. This is fine for typical computing conditions, but can cause significant performance issues for for games that constantly load sound files, textures and models. For this reason, configuring your ESET security product to scan only files containing executable code allows you to maximize your gaming performance with minimal risk. b) Adding protocol filtering exclusions for gaming Your ESET security product automatically detects programs like web browsers and email clients and scans them using the ESET kernel service (ekrn.exe) as an internal proxy server. This is fine for typical computing conditions, but can cause performance issues and loss of Internet connectivity in gaming-related applications that use network features, such as peer-to-peer services and update utilities. Note: A much more radical method is to disable the "Real-time File System Protection", but this is UNSAFE when online as threats could not be identified immediately. Do this only if you go offline!
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I hope this helps you,
XenonS