Tight timelines are something pretty common when working in the Agile model and also when the project planning is weak. Being efficient and productive is purely dependent on the experience of the designer and also the kind of deliverable which comes in the deadlines.
A designer can be efficient irrespective of the deadlines but the key is the output will vary. A given design will have to go through the process of brainstorming, ideating, relating it with the overall design structure which is defined, iterate and finally provide a design. When there are deadlines, designers will have less choice to work on the whole process to deliver a robust solution. This will result in something which is not a well-thought solution but maybe an interim one.
This is when the experience of the designer will count, as they think of the process only and give an output which will be well received.