Can you activate a planeswalker's loyalty ability at instant speed?

Loyalty abilities are a special category of activated abilities, but they come with a strict timing restriction. Under CR 606.3, a player may activate a loyalty ability only any time they have priority during their main phase while the stack is empty — in other words, at sorcery speed only.

Additionally, CR 606.4 states that a player may activate a loyalty ability of a planeswalker they control only once per turn. This means even if you somehow had priority at the right time, you cannot activate multiple loyalty abilities in the same turn.

This timing rule applies regardless of any effects that say you may "activate abilities as though it were an instant" unless that effect explicitly overrides the loyalty ability restriction. Generic speed-granting effects (like Leyline of Anticipation) do not allow loyalty abilities at instant speed, because CR 606.3 is a special rule that supersedes normal activated-ability timing.

Concrete example: Your opponent attacks you on their turn. You control Jace, the Mind Sculptor. Even though you have priority during combat, you cannot activate Jace's +2 Brainstorm ability to draw cards in response — you must wait until your own main phase when the stack is empty.

The only exception would be a card that specifically states it lets you activate loyalty abilities at any time (no such card currently exists in a widely relevant way), or an effect that grants you an additional main phase mid-turn with an empty stack.