The latest release of snapd, the engine that installs, manages, and configures snap apps, includes a couple of changes that improve the performance of the Steam snap specifically.
Snapd 2.65 sees the removal of “all AppArmor and seccomp restrictions to improve user experience”.
This doesn’t mean the Steam runs un-sandboxed, rather the Steam snap is more in control of its own containers (Steam is really more of a framework than app, with multiple components, parts, and so on).
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Anecdotally, the latest Steam snap release paired with snapd 2.65 is also reported to open faster than before, with launch times on-par with those of the DEB version. They’re also reported to be a couple seconds faster1 than the Steam Flatpak.
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Not strictly Steam related, but perhaps relevant from a gaming POV, snapd 2.65 also ships with improved snap-confine and OpenGL interface compatibility with NVIDIA drivers.