When people arrive on your website they expect it to be able to easily perform its core functional tasks, such as make a purchase or submit a form; it needs to be able to do what they expect it to do.
If your website doesn't perform as it should, you could be losing valuable traffic that may never return; why would a user come back if your website didn't work the first time they tried? First impressions count!
To test the speed of your website for free visit https://developers.google.com/speed/pagespeed/. This will give your website a score out of 100 and give you actionable insights in how to improve your site speed. You can also access this tool from Google Analytics, where you can see pages of your website ranked by slowest speed, and so on.
Testing external links is something an SEO expert can do for you by performing a link audit on your website. These are critically important if you're looking to replace your website, you need a link audit and plan in place to keep all of the Google ranking authority your existing site has when you replace it with the new site.
Consider how your webpages are categorised and organised. How each of your products or pieces of information is hierarchised will determine how easy or difficult it will be for users to find what they are looking for. Try using the blink test. Open your eyes for three seconds only on each landing page; is it clear what the page is about? Is it clear why the page exists? Is it clear where to go next?
Test the efficiency of your navigation by checking the number of clicks or actions you need to take to complete a task, such as a purchase or to find a certain product; each extra click is another chance for them to back out of a sale. Have you ever noticed the positioning of checkouts in supermarkets? It's no coincidence they're placed so prominently in the store, making it almost fool-proof for people to find them and make a purchase