While maintaining ranks and visibility on SERP, Google considers a website's load speed as a critical factor and might demote it if it scores lower than the threshold value assigned on those parameters in its algorithm.
Fortunately, it provides a free tool - Google pagespeed insights, which lets you know where your websites are lagging and what all areas inside the pages need a fix. But unfortunately, it doesn't explain to you the best ways of fixing them!!!
It is already a fact, that websites or stores built over Shopify for example, tends to slow down over time after additions of themes, high-resolution images, tools, etc. A slow eCommerce store translates into a higher bounce rate, and ultimately a lower conversion rate, which is a major bummer for any Shopify store owner.