Technical SEO matters because if your website is difficult to crawl, there is very little chance of having SEO success, as your website’s content would never reach Google’s index in the first place and thus would never be seen by searchers.
Additionally, having slow website speed can lead to users abandoning your website prematurely or not taking the desired actions when they are there.
Having a technically sound website is not just for improving rankings, it is also about improving your conversion funnel to help visitors eventually convert into customers.