If you notice your site is taking a long time to load, then you should first use PageSpeed Insights to see what can quickly be optimized (also check your file sizes, hosting server, NGINX and other common sources of slow speed, too.) And, if you've notice gradually fewer Organic Search visits, PageSpeed Insights might be able to help. As Google is reducing the prevalence of slow sites, you could be losing traffic. Speed up your page load times by finding what is blocking renders, slowing down contentful paints, and other critical page speed signals.
I think GTmetrix has some feedback that PageSpeed doesn't provide, but Google's tool is easier overall and has a better user experience; also offers compressed images, CSS, and javascript files if that's the case. Also, Google has extensive documentation to help you better understand what can be done to improve your grade.
Semrush's content and page optimisation tool is quite poor; it's not in the same bracket as Surfer. Frase is a good alternative from a cost perspective (if not paying the AI writing functions), it's cheaper, and Frase's keyword/topic heatmaps are quite unique. Sufer has a better UI and a shorter learning curve. It has an advantage in that it offers internal linking recommendations, which Frase lacks. Surfer also has limited competitor analysis (backlinks and page speed), which Frase does not. Both tools allow you to choose the number of competitors on the SERPS