perf: Avoid disk I/O during parsing (#972) #1351

Job	Run time
R-CMD-check / setup	40s
routine / routine	3m 50s
website / pkgdown	2m 39s
R-CMD-check / macOS-latest (4.4.2)	2m 48s
R-CMD-check / windows-latest (4.4.2)	3m 8s
R-CMD-check / ubuntu-latest (devel)	3m 26s
R-CMD-check / ubuntu-latest (4.4.2)	3m 22s
R-CMD-check / ubuntu-latest (4.3.3)	2m 43s
R-CMD-check / ubuntu-latest (4.2.3)	4m 39s
R-CMD-check / ubuntu-latest (4.1.3)	3m 15s
R-CMD-check / ubuntu-latest (4.0.5)	3m 3s
	33m 33s

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