Improve evaluation time of `roxygen` examples

[jasonratcliff]

Timing

System timing from devtools::check() runs using Gitpod cde branch contexts.

Gitpod

gitpod.io/#https://github.com/jasonratcliff/thesis/tree/main

• Commit d93eeafe5cbf128ffd36058293211f31b3e65ba1

─  checking examples ... [88s/87s] OK (1m 27.1s)
   Examples with CPU (user + system) or elapsed time > 5s
                            user system elapsed
   map_trait_distribution 18.162  1.318  18.058
   SpecimenMap            18.294  0.821  20.437
   layer_ggmap             6.569  0.296   6.532
   layer_borders           5.968  0.311   5.408
   layer_themes            5.418  0.314   4.976

Refactoring

gitpod.io/#https://github.com/jasonratcliff/thesis/tree/r/maps

• Commit 5e33a48f292a32f7cf875945463da1d52dff0924

─  checking examples ... [70s/71s] OK (1m 11s)
   Examples with CPU (user + system) or elapsed time > 5s
                            user system elapsed
   map_trait_distribution 16.046  1.301  16.140
   SpecimenMap             9.473  0.215  10.793
   layer_ggmap             5.732  0.195   5.518
   layer_borders           5.437  0.444   7.353
   layer_themes            4.920  0.201   4.474