readme: visually emphasize performance criteria difference #1868

Job	Run time
testfull-regex-automata	15m 0s
msrv	3m 44s
miri-regex-automata	15m 37s
rustfmt	15s
test (stable, ubuntu-latest, stable)	7m 14s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	4m 49s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	14m 27s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	18m 19s
test (beta, ubuntu-latest, beta)	6m 52s
test (nightly, ubuntu-latest, nightly)	7m 41s
test (macos, macos-latest, stable)	8m 18s
test (win-msvc, windows-latest, stable)	10m 10s
test (win-gnu, windows-latest, stable-x86_64-gnu)	6m 39s
testfull-regex	7m 2s
testfull-regex-capi	44s
testfull-regex-syntax	3m 19s
	2h 10m 10s

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