Why is D365 running so slow?

I hate to say it, but your system performance depends on many things. Here is my check list of possible issues, ranked from most to least likely (in my humble opinion):

• Synchronous workflows and plugins - You could test this (in a sandbox) by deactivating them and see if there is a difference in performance.
• Check your Settings/System Jobs to see if you have a lot of activity in there (especially failures). If you have years worth of system job data, I highly recommend you set up scheduled Bulk Record Deletion jobs.
• Data duplication rules – how many published rules do you have? Turn them off and see if there is a difference.
• Browser plugins (including antivirus)
• Browser cache – depends on the browser, but highly recommend clearing your cache daily because MS is updating the form rendering engine all the time.
• Is Auditing turned on?  It could play a small part in your overall performance.
• External apps, such as PowerApps, Flows, ClickDimensions that might add load to your database. If you set up an Azure application in a different region from your CRM instance, this could impact system performance (and unnecessary cost).
• Network throughput (Bandwidth) In a recent Gartner survey, 22% of IT leaders identified networking problems as the root cause for performance issues with Office 365. I have customers that have researched their connection speed between different MS data centers and have had unexpected results. Just because you are close to the data center does not mean it will be faster.
• Time of day – Sometimes I can see a difference in performance depending on the time of day, which is likely to be related to my upstream network throughput or the data center where my D365 instance is located. The only way I could test this would be to create a dedicated data circuit between my office and my data center.
• JavaScript embedded in your form that make a lot of API calls. Use the browsers built-in performance tool to see what is happening.
• Asynchronous workflows running will add workload to your database. For example, If you have imported 100k records which might kick off 200k async workflows, then users might expect to see slow performance while the workflows are hitting your system.
• Relevance Search and Text Analytics – crawls your site to index your data. This is adding load to your system.
• Legacy form rendering – you could try to experiment with this System Setting, but be warned – MS says that legacy form rendering will be turned off.
• Server Side Sync – do you have a lot of email coming in? Are new contacts created?
• Number of rollup queries – you would see how much activity is going on in the System Jobs Log (mentioned above). This would also include things like Goals.
• Cascading relationships – Look at each 1:N relationship and see what the “down-stream” effect is of your updates.