Yes the i3 just doesn't cut it at all, there's spikes and lags here and there.
Funny, I don't feel any "spikes and lags", maybe what you are doing is really demanding. Also have you update all the firmware and patches? sometimes its background downloading and indexing can cause a lag effect... just finish downloading and install all the updates, let it run for a while to allow indexing to complete(or you disable it)
My i3 sp3 is perfect for me for now. it is more than enough for my needs, yes, even just doing onenote with it is fine, no lag.. my priority is usually battery life over speed. Would love to have broadwell for battery life too. speed is not top in my list as if I need speed, I can get a powerful lappy or just use my desktop.
Also important not to have an expectation that this surface pro3 is to replace a full fledge laptop.. it is not.. this is a hybrid 2 in 1.. it is trying to compete with both an ipad and an MacBook air.. it is not a MacBook pro. if you are the so called power user, you should really get a MacBook pro or any alienware stuff.
Do note that the i3 has a lower idle frequency(0.57GHz vs 0.78GHz) which helps in using less power but may have a very very slightly slower reaction before steping up to the full base freq. You can try locking min to 100% and see if its still lagging to you.. if it does, no point getting the i5/8GB/256GB, you should just go for the i7.. as i5 and i3 is almost the same in both cpu and gpu, i7 is different, better in both cpu and gpu, since what you need is instant speed and not constant speed, overheating won't be your problem.
Or just get a normal tablet and give your zenbook a 2nd life. Maybe you have to wait till broadwell is out for surface to meet your needs...