Hi Oliver

As you mention that perfomance issues happen only on certain pages, first suspect would not be the hosting company. 
That said: in my opinion, DNN handles larger websites and/or with substantial traffic pretty well.
If you have a website with hundreds of portals or a database with millions of records, that's where you need to check out caching.

From that point on: bottleneck analysis. Any intel on the network tab in chrome? This would tell if it is due to images of 30MB or a API call with some external database or something internal. Or server monitoring, indicating that memory is just not enough?

Hello Oliver: 

When it's only some pages that are exhibiting performance issues, I'd suggest looking at the modules in use on those pages and determine what they do and what their dependencies are.  You may find that the module(s) have newer versions which may address some performance issues.  Upgrading can mitigate the problems. 

👆🏽 You should do this in the very least, including upgrading DNN if it's on an older version.  

In other cases, you may find that the module(s) in question may have a lot of older data that may be able to be archived and/or reindexed.  All of the standard .NET troubleshooting steps cn be done here.  

As for best practices, they can vary from site to site, but in general, I'd suggest the following: 

• Upgrade DNN and all custom/third-party extensions on the site.  
• Uninstall any extensions from DNN that you're not using. 
• Ensure that any necessary database maintenance is occurring at an acceptable interval (including regularly rebuilding indexes).  
• Archive any older log files.   
• Ensure that the hosting environment has all necessary updates.  
• Avoid shared hosting, but nearly any other hosting plan could be sufficient.  Newer versions of DNN can function quite well with minimal resources, except during installation and upgrade of DNN itself.  
• If you have any custom modules, ensure they are reviewed and rebuilt using all of the most current dependencies.  (Including updating DNN dependencies to 9.11.0+ and any removed API endpoints.) 
• In DNN itself, enable caching and ensure it's configured to use Heavy Caching.  Do not use memory caching if you're using any SPA modules/features. Page-level caching is better for these purposes.  
• Deploy and configure a WAF in front of your site, such as CloudFlare.  
• If your database and website are hosted from the same server, review the resources to be sure that you're not hitting the upper resource limits.  

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Well, we still got some good content out of it for the community.   😎