Yup. The speed depends more on the dataset
You didn't tell us whether .id was a unique index for either a or b (or both)?
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A stupid despot may constrain his slaves with iron chains; but a true politician binds them even more strongly by the chain of their own ideas;...despair and time eat away the bonds of iron and steel, but they are powerless against the habitual union of ideas, they can only tighten it still more; and on the soft fibres of the brain is founded the unshakable base of the soundest of Empires."
Jacques Servan, 1767