I think OP is just alluding to the fact that Postgres needs to do less work to go from secondary index to table data, since the tid is a direct pointer to the exact page and slotted entry while MySQL needs a b-tree walk.

> primary key index will be mostly cached so the cost of the indirection is much smaller than it may first appear

Not sure I follow. If it's in-memory you save having to read from disk, but you still have to walk the b-tree to go from PK to data.