It’s based on the search of bottlenecks and critical paths in your operations, so you can improve the process as a whole as any improvement in that point will enhance the total outcome. The idea comes from factory lines where a machine or an activity is delaying the whole line and preventing you from manufacturing more tomato soap cans. 

Your team probably (I hope) follows a sequence of actions: First user story is refined, then developed, tested, reviewed by colleagues, after that a pull request is triggered, software is merged, then tested in an integration environment, and finally reviewed and approved by Product Owner. What is constraining you from delivering more value at the end each iteration? Maybe your refinement session is not long or frequent enough to get all the stories refined, or perhaps Tech Lead is too busy for the huge amount of pull requests she is receiving, and they start to pile up in the repository tool. Do you need another pull request approver? 

Analyze the series of steps your team follows. Try to tweak your proceedings a little bit every time. You’ll make significant progress in your cycle time.