Efficiency comes when technique meets strength, balance, and flexibility. It is never easy to maintain proper form, especially at the end of the workout and you are on your last rep. When your body becomes fatigued (same goes true with the mind as well), it becomes easy to want to take shortcuts and to just want to go through the motions. However, it is during these times that it becomes even more important to be diligent on keeping proper form and technique. Drilling it in when your body is tired is the perfect time to bring your fitness to the next level.
On the flip side, continuing to exercise with poor technique will cause inefficient movements and may lead to injuries. Continue to maintain good technique and exercises that were once tough will become easier to execute.