Join and merge benchmarks.

Is there benchmark for join and merge operation on indexed columns? According to official Dask performance guide, this operation should be fast compared to pandas, but when benchmarking on my data, the speed up when using 8 partitions was ~7% which was quite disappointing, and I could not find any benchmarks for this.

No files, tests, or entry points are named. Start by reviewing the repository's existing asv benchmark structure and the official Dask performance guide, then identify how indexed join and merge cases are represented. Done means adding benchmarks that measure these operations and make the results comparable with pandas.

Written by the indexing model from the issue text.