Slow rebuild (single threaded)

[taki-sahli]

Hello,
The nixos-rebuild command tooks so much time to build my configuration.
In btop, the command uses only one thread.
Even I add the --max-jobs and --cores flags, but no success.
One built unit takes min 1 minute, so for this task, it needs 3 hours to complete 🤯.

Can you help me fix this issue.
Thank you.