Full disclosure: I work for Qubit who published this white paper.
I see a lot of this kind of testing going on in the industry and it's frustrating. A/B testing can be a massive tool for your business if it's done right but obviously if you only wait for 2-3 conversions, you're not learning much... "Good" to hear that other people feel the same way!
Well it depends on your conversion rate. But assuming it's ~few percent, then yes it will be hard to measure anything other than very very large effects in conversion rate unless you're willing to wait quite a long time.
You can measure more upper-funnel things though like I said in the GP, which can be very helpful, especially in combination with qualitative feedback, although this depends on what exactly your business is...
I see a lot of this kind of testing going on in the industry and it's frustrating. A/B testing can be a massive tool for your business if it's done right but obviously if you only wait for 2-3 conversions, you're not learning much... "Good" to hear that other people feel the same way!