I had worked on a large project in after effects 2 weeks ago on my Mac pro 16 inch 2019. The project was large so the previews slowed down considerably as expected. But ever since i worked on that project every new project i create in after effects no matter how small has slowed down considerably to the point where i cant even preview without AE giving me a low memory warning. I have adjusted the memory and performance, i have purged the memory and cleared the cache and tried virtually everything i can find on youtube and nothing changes. It was working perfectly fine since working on that large project and ever since its so slow i can barely work on anything new.
Try resetting your preferences. I'm seeing a lot of performance problems related to corrupted preferences that can't be fixed by just changing the settings. You need to hold down Shif + Alt/Option + Ctrl/Cmnd when you open AE and reset the preferences.