Haven't been on AutoCAD for a while. maybe try GC command to free up memory might help.
use mem at the start when its nice and fast and when it is starting to slow down.
https://help.autodesk.com/view/ACDLT/2026/ENU/?caas=caas/documentation/ACD/2014/ENU/files/GUID-F4AEB953-2117-4BF2-8056-EA1384AC3FFF-htm.html