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Benchmarks are a bit weird: simple units are faster, as expected, but quantity initialization has slowed down? Will try to rerun, especially given the large variance, but for the record:

| Change   | Before [b266a404]    | After [b02b73a0]    |   Ratio | Benchmark (Parameter)             |
|----------|----------------------|---------------------|---------|-----------------------------------|
| -        | 2.46±0.01μs          | 1.85±0.01μs         |    0.75 | units.time_very_simple_unit_parse |
| -        | 123±70μs             | 48.6±0.2μs          |    0.39 | units.time_quantity_times_unit    |
| Change   | Before [b266a404]    | After [b02b73a0]    |   Ratio | Benchmark (Parameter)                |
|----------|----------------------|---------------------|---------|--------------------------------------|
| +        | 38.6±0.2μs           | 126±90μs            |    3.25 | units.time_quantity_creation         |
| +        | 97.9±1μs             | 234±100μs           |    2.39 | units.time_quantity_init_array       |
| +        | 168±1μs              | 309±100μs           |    1.84 | units.time_quantity_array_conversion |

EDIT: OK, the rerun of the benchmarks only shows the expected improvement (plus a still flaky run on main):

| Change   | Before [b266a404]    | After [b02b73a0]    |   Ratio | Benchmark (Parameter)             |
|----------|----------------------|---------------------|---------|-----------------------------------|
| -        | 2.46±0μs             | 1.89±0.02μs         |    0.77 | units.time_very_simple_unit_parse |
| -        | 139±80μs             | 47.5±0.1μs          |    0.34 | units.time_quantity_times_unit    |

Hmm, looking at your PR again, I think it is really equally simple (after the other changes made), and does cover the FITS case. So, let me close this one in favour of #17004.