No, a sitting POTUS 'will' not be criminally prosecuted for anything whilst in office, not even murder. Theoretically he'd have to be impeached and removed from office first. That's what saved his bacon from criminal prosecution during his first term. All the prosecutions took place after he left office.
There's no specific law that says they can't be prosecuted whilst in office, but it's long standing DOJ policy (OLC memoranda 1973 & 2000) and let's face it, no sitting POTUS (GOP or Dem) is going to undo that policy, although I can see SCROTUS undoing it as one of his last official acts especially if a Dem wins the 2028 election or if one of his 'enemies' in the GOP does in the hopes it will cause them problems.
The 'in the performance of his duties' regarding misdeeds in office was the huge SCOTUS decision wrt to presidential immunity (Trump vs USA issued 7/1/24). It provides that a POTUS can NEVER be prosecuted for anything if it can be determined it was in the 'performance of his office/duties'. Prior to that decision a POTUS could be tried for crimes in office, that's one of the reasons, if not THE reason, why Nixon 'took the deal' and resigned over Watergate. This decision is what set SCROTUS up for all the shit he's pulling now; prosecution of his 'enemies', dirty deals with Russia & Saudi, and the many moneymaking schemes he and his family are doing. He's banking on everything being determined to be 'in the performance of his duties', which is a pretty loose term.