Can you tell me how to speed up the preview in After Effects 26?

I lost 2 repeat customers because i couldn't render fast enough or five them previews after i had made changes. I went on a 7 year quest, talked to other vfx artists, tried every new suggestion from dozens of youtubers even, swapped components, eventually got a new computer 7 times more powerful, and... then i lost the second client. Got some use out of reducing preview resolution or correcting errors such as caching to the wrong drive or not using MP4 files, but nothing grand.

 So from my personal experience-

Could you please try going to Edit/Purge all memory and disk cache and try again after completion?

Also giving us your system specifications would be helpful.