Most likely caused by compiler flags, for example, nixpkgs does not enable LTO by default, but Arch Linux, Ubuntu, Fedora, OpenSUSE and more enable it by default.
More examples:
for reproducibility disable pgo for clang- Nix compiled Python is slower
- firefox is slower than firefox-bin
Issue 3 has been resolved, but 1 and 2 and possibly other packages are slower on NixOS than other distributions
I tired KDE Linux in a virtual machine, and it’s even faster than my host os (NixOS).
This might be caused by the explosion of env vars like XDG_DATA_DIRS, though most of the time, it should only slow down startup.
EDIT: I think “disable pgo for clang” is not entirely correct, pgo is not enabled for clang in most other distros either. Python is indeed slower.