I feel it depends on your AEM set up too. Below documentation from the Adobe gives an idea on the performance based on the set up you might have
https://experienceleague.adobe.com/docs/experience-manager-64/deploying/configuring/performance-guidelines.html?lang=en
Also in the same post https://experienceleaguecommunities.adobe.com/t5/adobe-experience-manager/aem-author-heap-size-error/td-p/400184 given by @jagadeesh_prakash
please check the reply from @Jorg_Hoh reply on memory issue
To understand the root cause for slowness, I would use the following approach:
* enable Garbage Collection logging on the JVM and monitor the GC behavior.
* quantify "slowness": Determine "slow" requests and make it actionable (what makes a slow request "slow"? Response times over 1 second?)
* check the periodic threaddump and search for patterns; correlate it with observed slowness (slow requests). It's definitely best if you could reproduce this in a lower environment and attach a profile to it to understand where the time is spend.