Explore options to speed up `coverage` build

The `coverage` build seems to be the slowest build at the moment. It seems like the merging all the coverage results is single threaded and takes quite a bit of time:

```
Step #2: ==> 🕑 merging done in 674.070 seconds
```

Start by reproducing the coverage build and measuring the merge step that reported 674.070 seconds. Inspect how coverage results are merged and determine whether a practical speedup path exists. Done means documenting or implementing a supported approach that reduces the merge time and validating the result against the coverage build.

Written by the indexing model from the issue text.