As a merchant, you are rightfully worried about your Site Speed, but many turn to Googles PageSpeed Insights for testing. The problem is that this tool does nothing more than instil fear and anxiety. Despite the name, it's not a tool to test your Page Speed!
What makes you say that?
The proof is in the pudding! First, ask yourself, what do you want to know if you are using a tool to test your page speed? You want to know if your website is slow for your customers of course!
So let's look at a couple results from PageSpeed Insights that illustrate my point perfectly, and shows why you should not use it for this purpose.
Here is Apple vs a Shopify hosted website:
Apple is a billion dollar company with their own data centres and thousands of people running their infrastructure and websites. Nobody would consider their website as slow, but these results sure make it look like it's not performing well.
If Apple only gets 3 more performance points vs a website hosted on Shopify, what hope is there for merchants to get a good score?
You can see how this kind of report would scare the crap out of most merchants and make them think their website is incredibly slow. Yes, you can increase this score, but it may not actually speed up your website by much.
What should you use
There's a number of tools that can be used for testing page speed that won't give you palpitations as soon as you run a scan.
GTMetrix is not only a great tool for testing site speed, but it also gives you the opportunity to compare directly against other sites. Some may find this a little overwhelming still, but if you find the Page Speed section, you're shooting for under 3sec overall load time.
I also suggest people look at using Pingdom for this purpose too. While they don't have a compare option, they do have an elegant and simple interface that can really help you understand what is really slowing down their website (Spoiler: It's usually Images/Videos and Scripts). Page Speed is displayed front and center with Pingdom.
In closing
While PageSpeed Insights is a great tool for developers, if you want to actually see what you page speed is, I would use one of the other tools mentioned above.
There are developers that specialize in increasing PageSpeed Insight scores if that's what a merchant wants, but I would suggest looking at some of the things outlined with the other tools before getting into something like this.