I am facing the same issue. I have checked old projects which had perfect playback of animations during editing. They are not performing the same. I thought it might be an issue with CPU or RAM. I have also tried to give AP higher process priority and stopped all antivirus SW and anything else that could affect performance. Would like to know if there is anyway to fix this sudden lag issue.
I unchecked the use software rendering instead of GPU rendering. Restarted AP and the animations have become smooth again. Updating this post in case this is the correct fix and if it can help others.