@Dezis from a website perspective, the product page is usually the most influential element.
The biggest conversion killers I see are:
Unclear value proposition (people don’t understand what makes your product different)
Poor or missing product descriptions
Not addressing customer objections upfront
Bad imagery (or not enough of it)
Not showing people what their life looks like after buying your product
If I could only optimize three parts of a Shopify store, I’d focus my initial efforts on the product page. That’s where most traffic spends their time anyway.
Make sure people 1) clearly understand what you’re selling, 2) why it matters to them, and 3) what makes it different. Address the obvious objections before they have to ask.
On your last question about determining whether a conversion problem is caused by the store or the traffic source - that’s tricky.
As long as foundational elements are in place (everything I mentioned above), you definitely need to pay attention to your traffic sources and creatives. If one source performs way better than others, that’s usually a signal that something isn’t working for a specific audience.
Business size matters here too. The more you scale and bring paid traffic, the more likely you are to attract unqualified visitors. There’s no single way to diagnose this, but what works well in my experience is running exit intent polls with tools like Lucky Orange or Hotjar.
Literally just ask people: “What’s preventing you from completing a purchase today?”
That activity is usually very insightful to understand some of the blockers.