In brief: The market of "modified" Windows versions has been around for a while. AtlasOS strives to be the "best" version of Windows 10 for gamers - or for hardware platforms which are a bit out of date to properly run a modern OS.
AtlasOS is an "open source" modification of Windows 10, a redesign of the most popular PC operating system which, according to its creators, has been specifically "designed for gamers." Atlas users can enjoy higher frame rates and lowered input delay & latency, according to developers. The OS requirements, however, are great for both people on low-end systems and high-end gaming machines.
As presented on the project's official site, the main features of AtlasOS include a reduced number of running processes to lower computing stress on the CPU, while lowering system latency at the same time; more storage space, as the OS ISO is pretty small compared to Windows official images; a focus on privacy because Windows telemetry has been disabled; open source transparency, so users and coders can check what kind of modifications AtlasOS actually brings to Windows 10.
When compared to a "stock" Windows 10 21H1 version, the latest release of AtlasOS (0.5.2) cuts the number of running processes from 185 to around 35; RAM usage is greatly lowered as well, going from 1.5GB to 600MB. Process latency, which AtlasOS creators call a "sticky topic," also goes down from 3.09 to 2.55.
Atlas removes all the "negative drawbacks of Windows" which could adversely affect gaming performance, the project's official GitHub page says, while improving system and network latency and input lag at the same time. Removing tracking features from Windows 10 seems like a very good idea anyway, but where AtlasOS follows a questionable path is when it disables many security features which are described "bloatware."
AtlasOS disables mitigations against hardware security flaws in modern Intel (and AMD) processors like Spectre and Meltdown, following the principle that "if a security mitigation measure decreases performance, it will be disabled." As they are not crucial for gaming, standard (security) features like Trusted Platform Module (TPM), BitLocker, Windows Defender, Voice Recognition, Restore Points & System Reset are all removed as well.
As for the open-source nature of the Atlas Project, the main tool currently used to create the mod is NTLite - which is a closed-source software. AtlasOS creators say they want to eventually provide a script to help users build a custom ISO of the OS at no cost.
The usefulness of AtlasOS, all things considered, is questionable to say the least: gamers and enthusiasts tend to purchase the most powerful hardware they can (GeForce RTX 4080 fiasco aside), so the promise to use Windows 10 on a "potato PC" is not much of an option. AtlasOS offers fewer features, less compatibility and less security compared to Windows 10, which doesn't seem to be a great choice for both gamers and common users as well.