Multiprocessing to speed reporting?

[juandiegopalomino]

Is your feature request related to a problem? Please describe. So I have like a huge project to deal with, with hundreds of files and it takes over 2 minutes to generate the xml report and over 4 minutes to generate the html report.

Describe the solution you'd like Seeing as how, at least for html, there's a loop working over what appears to be autonomous pieces of data (and writes to separate locations), would it not make sense to use the multiprocessing library and cut down time based on the number of cpus available? https://github.com/nedbat/coveragepy/blob/aa62abd5ff33926f44fe4ec9e985ed3d72ea1f9d/coverage/html.py#L230

Describe alternatives you've considered I was thinking of maybe writing a tool in go which performs the html generation after reading the sqlite file, but that is silly as opposed to helping fix the original tool.

Additional context This is a great project, thank you for the hard work!