The power you need varies with the cube of your speed, because of wind resistance. So travelling at 30 kph on a flat road requires close to 3.5 times as much power as does 20 kph.

In that context, a 2% loss in efficiency in power transfer is going to be lost in the noise. Looking at the figures on a bike calculator, a loss of 2% in drivetrain efficiency is going to mean I need about 4 watts more to maintain 30kph. Maintaining the same power would cost me about 0.3 kph. If you're racing competitive time trials, significant. Commuting, negligible.