If you change offhand out-of-combat and then start swinging at a target, your swing timers get reset as expected, and the offhand speed reported by the WoW API for that swing is correct.

If you do the same in-combat, the offhand's initial swing will be set at the speed of your mainhand weapon, possibly a little higher because the mainhand always swings first. The WoW UnitAttackSpeed endpoint does not account for this, and will just return the offhand speed for that swing.

To get an accurate timer on the initial swing for an in-combat offhand weapon change will necessitate some additional logic when we detect a weapon change.

I haven't tested the behaviour for when the offhand is slower than the mainhand weapon, but I can do this at some point to make sure we understand it properly.

This one need a round of test for sure. It will be interesting to also test what happen if you switch a offhand weapon from a slower to a quicker speed.