We have a story "Scale service to X 000 transactions per second". It's unclear how to do that and how much time will it take. Usually in this case you can define a Spike to find out what obstacles you will face during implementation, what steps have to be done etc., so on the next Sprint planning you have enough information to define tasks properly and estimate the story.

However in case of our story it's iterative process. You run load tests, see that goal is not reached, profile, optimize something, run load tests again, profile, optimize, ... - research-implement, research-implement many times.

How to handle this type of stories?