Many things. After thinking for a while... Seems quite a few things besides heredity can produce slow thinking - most associated with global metabolic process or intracranial pathology... Any abnormality of electrolytes but especially ca, glucose, marked dehydration or osmolal states, acidosis/ alkalosis, hypoxia, poisonings or intoxications like alcohol, elevated icp, tumor, encephalitis, meningitis, etc.
Tired? Medication? If you are an otherwise healthy 34 year old, you should not worry about atypical or premature cognitive function decline. If you have medical issues such as diabetes, renal insufficiency, blood disorder and others, a "slow down" is a red flag and you need to check in with your doctor soon.