Honestly, part of the upgrade process should be to remove all unused extensions and upgrade all others every time you upgrade. This is one of the most common issues I see in some sites that cause issues.
Once that's done, maybe revisit your performance settings and even make sure you have a WAF (website application firewall, like CloudFlare) to help with performance. However, DNN should be performing quite well without it. The main benefit besides the CDN features is the additional protection for your website.
Next, I'd suggest comparing your web.config to a clean web.config of the same DNN version. This can help you find other missing updates from previous upgrades and sometimes fixes many issues.
Here's a presentation that illustrates our upgrade process from last year's DNN Summit conference.
Here's a link to this year's conference too. The early bird rate is almost over.