CLN/ASV: reduce size of join_merge.JoinIndex
The existing `join_merge.JoinIndex.time_left_outer_join_index` ASV joins two frames with indexes containing lots of duplicates resulting in a large result (~25m rows). This is resulting in long (and inconsistent?) run times (700-3000 ms):

<img width="304" alt="Screen Shot 2023-07-16 at 12 36 34 PM" src="https://github.com/pandas-dev/pandas/assets/8519523/fd0fbd49-581b-4680-978c-3fb71a7b1560">

This PR reduces the size of the frames being joined such that the result is ~250,000 rows and the runtime is < 20ms. 

Nice! Happy to have more of these benchmark tuning @lukemanley 