: Declare war on the Ottomans and park a ship (even a small sloop) in the strait between Europe and Anatolia. This prevents the AI from attempting the crossing, which often resolves the lag. Capture Istanbul
Even with mods, your PC can choke. Do this last: empire total war darthmod ottoman turn lag fix
A community-created fix has been developed to specifically address the Ottoman turn lag issue: : Declare war on the Ottomans and park