What version of ZBrush are you running? When 2021 was first released there were a number of performance issues related to the use of Windows Ink. To my knowledge, most of these issues were addressed in the 2021.1 update. If you are not running the most up to date version of the program, please upgrade.
Keep in mind that turning on polyframe mode will reduce performance on very dense meshes, but I wouldn’t think to the degree that you report.
If your program version is up to date and you are experiencing this, you should report this issue to Pixologic Support.