A perfectly good Economist article about food and brain function includes the following:
Many studies suggest that diets which are rich in trans- and saturated fatty acids, such as those containing a lot of deep-fried foods and butter, have bad effects on cognition. Rodents put on such diets show declines in cognitive performance within weeks.
Whereas I found butter improved my cognitive performance within a day. And pork fat improved my sleep within a day. On the other hand, I wouldn’t be surprised if foods deep-fried in plant oils, such as corn oil, are bad for the brain.