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.
SEOquake is a great tool to use alongside Semrush with the quick data that it provides on the search results page it's helpful to make quick decisions. The ability to change the SERP locale is also really nice as it helps us analyze the search results page from different locations.
Gives a ton of SEO info in one area. I usually have to use a few different Chrome extensions that show different areas to get the full picture that this tool gives you up front.
It gives you info on ways to improve the infrastructure of your website.
The high-level parameters are nice to see, but for a new-to-the-tool person, they are confusing and don't provide a lot of explanation, unfortunately.
The keyword density section is somewhat helpful but gives the density in a percentage without rationale as to what an average percentage is or what the website should aim for.
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.
SEOquake provides more information regarding backlinks and traffic in general whereas other tools focus more on keywords and their search volume. The backlinks information it provides is also really useful in making quick decisions on which site we can focus on more when it comes to searching for backlink opportunities.