Hello
None of these medicines would deter your cognition. Infact, anxiety affects memory, concentration, a lot more.
It is likely that your worry (anxiety) would make you your productivity much lesser as you would require to focus.
Also please notice, the effects of all medicines takes about an average of 2-3 weeks to show. Further, there is a reduction in percentage of anxiety not a 0-100 change.
All the best for you exam.
Next Steps
Trust your doctor. Medicines take a little while to work.
Health Tips
Try to take a deep breath when very anxious. Once your exam is over - Try therapy for anxiety in combination to understand n treat the source of your anxiety.