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.
SEOTesting.com is great in monitoring the results of implemented SEO optimisations. SEOTesting.com also has extensive content analysis reports that make it really easy to find long hanging fruits for content optimisation. However, I would not recommend SEOTesting.com for rank tracking and it has no tools for technical SEO. But if you pair SEOTesting.com with some other SEO tools of your preference, you get a really valuable toolset with decent pricing.
SEOTesting.com is NOT all-in-one SEO tool. But what it does, it does really well.
Rank Tracking requires to fill landing page per keyword. That doesn't make sense as the desired landing page is not always the best ranking landing page for each keyword.
Dashboards timelines should have some options to select only the desired timeframe. Then it would be sometimes easier to look at only the last two months for example.
The account person is quick to respond, though issues reported required support of other teams with no SLA. The data is easily comprehensible and the platform is incredibly versatile. Sometimes scanning problems take weeks to diagnose and fix. The learning curve for SEOTesting.com is minimal
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.
SEOTesting.com is a very specific, focused too. It's not "better than" the other tools. It does something they don't do well: gets data from your SEO results out into an easily-read, easily-reported format so you can 1. learn from what's working and what's not 2. show your team what success & failure look like 3. demonstrate to your boss/client the short-run benefits of investing in SEO Search Console has all the data in it. In fact, SEOTesting.com gets its data from Search Console. But you are going to waste your time and your colleagues time if you try to monitor granular, week-by-week progress for different pages in Search Console. SEOTesting is awesome at that. Use Ahrefs and/or Semrush to get ideas on how to improve your website's content and tech SEO. Then make the changes. Then use SEOTesting.com to find out if it worked!