Speed up vis_drake_graph() for large numbers of dynamic sub-targets

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.
• Advanced users: verify that the bottleneck still persists in the current development version (i.e. remotes::install_github("ropensci/drake")) and mention the SHA-1 hash of the Git commit you install.

Description

One of my real-life drake workflows has 32000 dynamic sub-targets. vis_drake_graph() is slow for this, and it spends most of its time reading build times from the cache (see below). Rather than read the sub-target build times, we should just read the build time of the whole dynamic target and make a note of how many jobs were used at the time.

Benchmarks

I cannot show the code because it is confidential, but here is a flame graph.

[wlandau]

0050de9 should do it, but I need to test on the large project to be sure.

[wlandau]

Indeed, vis_drake_graph() is several orders of magnitude faster on the full project now.