27 times out of 11,000 proposed amendments, and as you point out, the first ten happened in bulk. I doubt that the founders planned for 2% of the population (as conservative Antonin Scalia pointed out) being able to block an amendment.
Unless you can tell something about the distribution of the quality of constitutional proposals, then the percentage is meaningless. Besides, it's designed as a document that is a scaffolding for the rest of law, and shouldn't require a high rate of change.