Very important. Even if you disregard the apparent disadvantages to conversion rate if your website is slow, it is also important for Google – both for your SEM and SEO.
It is hard to do an objective test for speed; you might be sitting on a fiber-connection on the biggest workstation the world has ever seen and think that your website is lightning fast — but Google might not have the same opinion.
To make things more complicated, it’s not just about speed; it’s about user experience (in terms of clickable objects being too close), and it’s about the size of what is loaded.
If your site loads fast, in terms of what is needed to be loaded for a user to start navigating but then continues to load pictures that might not be seen, that’ll be a problem.
In some sense, Google’s speed is not the actual speed of a web page but more focused on user experience.
To make things easier, Google has a tool called PageSpeed Insights, where you can test your site and get a score from 0-100, one for mobile and one for desktop. The tool also gives you a list of recommendations to improve your speed.
You can find the tool here: https://developers.google.com/speed/pagespeed/insights/
Click the link and enter your URL to get the PageSpeed Insight for that specific URL.
If there are enough user data, 30-days running Google will give you a score on the whole site, called “Origin summary,” where you get a score from Slow – Moderate – Fast.