The way we perceive duration is subjective.
It's inevitable: we'll all have to wait for things sometime. When talking websites, this can mean loading or calculation times. Although you'll probably do everything to get a fast website to prevent people from waiting, sometimes you can't get around it. But that doesn't mean you can't make the waiting more pleasant, or at least less of a burden. What you do with that period of time effects how your visitors experience how long it actually takes.
Example: You probably know those preloader images that cycle around until everything is loaded onto your screen. Because we tend to count cycles (and not the actual time it takes), using slower/less cycles makes us perceive the loading as faster. Another thing you can do with loading time is preloading relevant content or a fun distraction to keep people busy and not just waiting.
Recently I've seen some (often absolute) statements going around, generally in the line of "open source commerce platforms are a terrible idea". Now of course different solutions always have different pros and cons.
A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. I've created a version of this chart/pyramid applied to CRO which you can see below. It contains the options we have as optimizers and tools and methods we often use to gather data.