Offhand ias and wsm (weapon speed modifier) have no bearing on trap laying speed.
Therefore 30 ias on mainhand and 99999999 ias offhand is still only 30ias.
But the amount of ias you need depends on the wsm of your main hand claw. As an example, greater talon has a wsm of -30 and suwayyah has a wsm of 0. Where the bigger the negative number is, the faster it is and therefore the lower amount of ias you need on weapon or from gear sources (not offhand) to hit the respective break points