Hi,
First of all, you should be aware that SEMRush changed their algorithm a week or so ago and a client of mine also saw the same issue. They have really set the bar low when it comes to showing this factor now, to the point I don't agree with it.
However, I would use this as a signal just to allow you to see if the content is lacking a little on the page, rather than worrying if it will affect SEO. Of course, if you have a page with no content, this is bad for both usability and SEO, but text / HTML ratio isn't a direct ranking signal. See it as more of an indicator to see if the code can be cleaned and to help increase page load times.
There is never any harm in trying to speed up your pages either. If you think that some of the code is slowing things down, then look at how this can be improved. You can always do more to better the user experience, and with that will come indirect SEO improvements.
Write amazing content for your pages (where possible) and don't worry too much about text / HTML ratio. I know which one Google will be paying more attention to 
-Andy