Because of T190079 we now ask users in source mode whether they'd like their paste of formatted content to be converted to plain text or to wikitext. Although this is useful, it adds a bit of friction into any paste-heavy workflow, particularly if the task the editor is performing always needs the same answer to the modal.
We do have a keyboard shortcut for forcing plaintext pastes (ctrl-shift-v), but no equivalent for forcing the wikitext. We could add a similar shortcut (ctrl-alt-v?), to mean "paste and convert to wikitext if needed".
Alternately, we could put a checkbox in the modal, as "remember my choice for this edit session" (i.e. sessionStorage). I don't think a flat wiki-level preference would be appropriate, as it's fairly contextual to the task the editor is performing -- they can know that everything they want to do this time needs to be converted, but the next article they edit could be entirely different.