Any foods which are phlegm or damp producing will hamper the ability of your mind to operate quickly. These are things such as high fatty or fried foods, excessively sweet foods, cold foods like ice cream and anything else that is generally very heavy if eaten too often and in large amounts.
The brain generally benefits from use and not a lack thereof - so practicing memory functions is the most likely to produce a result, not just depending on some magic food.
Fish and seafood in general contains a great deal of essential fatty acids as do fruits, vegetables and grains like avocado or flaxseed.
Other foods which do not directly contribute but instead, help to remove the damp and phlegm which clouds the mind from its original function include bitter melon, adzuki beans, mackerel or ginger.
Strong adaptogenic herbs which may assist your mental state when used correctly include basil in large amounts, preferably uncooked, ginseng (use with caution - best when there is concurrent poor appetite, loose stools and fatigue together with poor memory), Angelica (best when there is concurrent dizziness, heart palpitations, poor short term memory, poor sleep and generalized fatigue), horny goat weed (use with caution - effective only when there is lower back pain worse with cold, exhaustion, coldness of the knees and elbows, low sexual drive, frequent pale urination and nocturia, frequent infections or illness and possibly edema of the feet of ankles).