Some thoughts:
- Using (multiple) Distance Maps with either many objects or heavy geometry can result is slower scene performance if the material editor is open and is displaying these maps or materials using them
- World space modifiers on many objects can slow down a scene due to mesh data being transferred forth and back
- In general, Slate editor and browser both are mostly slower compared to the legacy material editor and browser, closing it may help
- Try to hide the ribbon toolbar within Max UI. It can slow down Max, and for some users, bad viewport performance improves if you reset it: right click > Ribbon configuration > reset Ribbon to default
- Check if your scene has an excessive number of animation layers or empty tracks - some cleaner scripts can take care of these. In some cases, scenes tend to introduce crazy number of tracks for some reason, which bloats file sizes and affects performance
- are you using lots of 3rd party assets or geometry produced by plugins? Make sure to inspect these, maybe they are the culprit?
Have you tried to identify the source of the slowdown?