See e.g. [this at baeldung.com](https://www.baeldung.com/linux/bashrc-vs-bash-profile-vs-profile)

Basically, in an interactive **login shell**
1. `/etc/profile
2. User profile
    1. `~/.bash_profile` if present; else
    3. `~/.bash_login` if present; else
    3. `~/.profile` if present
but not `.bashrc`. Thus usually, `.bashrc` is sourced by the `.profile`.

The issue is the overhead in setting everything up, since `.bashrc` is run
every time a new shell is opened, and most variables can be inherited from the login shell.