I always make sure that we are actively using Bing and Google. There is a tradeoff in your time when you use more tools, but there's also benefit in multiple perspectives. And don't forget that Bing is a very viable search engine that receives 500 million search queries each month. You may find that your business benefits from a relatively small amount of attention paid to Bing because search ranking there is easier (not easy, easier).
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.
The user-interface isn't as friendly or appealing as Google's - very bland overall.
The way they present their Page Traffic report is a big turn-off. It's basically just a grid of numbers and arrows which is hard to interpret at a high level.
I think that Bing does a good job of being a competitive product in a space that has very basic and insanely complex products. While Bing isn't pretty, it gets the job done and also gets you the right tools in a simple and straight forward backend UI. The ability to place Bing ads, track SEO and see how your site is performing, all from one backend interface is awesome!
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.