Does Teferi, Time Raveler's static ability prevent opponents from casting spells at instant speed during their own turn?

Teferi, Time Raveler's static ability reads: 'Each opponent can cast spells only any time they could cast a sorcery.' This restricts when opponents may cast spells, not who is allowed to cast them. Casting at sorcery speed means casting during your own main phase when the stack is empty and you have priority (CR 307.1).

So on an opponent's own turn, during their main phase, with the stack empty, they absolutely can still cast spells — that is exactly what 'sorcery speed' means. Teferi's restriction only removes their ability to cast spells during other players' turns, during combat, in response to other spells or abilities, or at any other instant-speed window (CR 116.1, 307.1).

What Teferi does prevent is opponents casting spells at instant speed — for example, casting a counterspell in response to your spell, flashing in a creature at end of turn, or activating abilities that involve casting spells outside of sorcery timing. It also notably does not restrict you (the Teferi controller) at all.

Concrete example: Your opponent wants to cast Opt (an instant) on your end step. With Teferi on the battlefield, they cannot — that window is only available at instant speed. However, on their own next turn during their main phase with the stack empty, they may cast Opt just fine, because that timing satisfies sorcery-speed requirements (CR 307.1).

Note that some cards have abilities that let them be cast 'as though they had flash' or similar; Teferi's restriction overrides such permissions for opponents, since CR 601.3 clarifies that timing permissions don't override Teferi's blanket restriction to sorcery timing.