Summary

Fix slight performance regression of #898.

I only noticed this after upgrading to 22.1.0, but the PR i made a while back to speedup the instantiation of frozen classes did so, but at a cost of not scaling correctly the more attributes that need to be set

Didnt want to just revert the commit, so played around a bit and found some optimizations. which are in this pr. They include (partially) going back to some of the old code, but with some optimizations that makes it both scale better and perform better in simple tests, ending up being a win win. Sorry for the slight performance regression this issue caused. It didnt cross to me it wouldnt scale well.

$ pyperf timeit --rigorous -s "import attr; C = attr.make_class('C', ['x', 'y', 'z'], slots=True, frozen=True)" "C(1, 2, 3)"
.........................................
Mean +- std dev: 499 ns +- 26 ns

$ pyperf timeit --rigorous -s "import attr; C = attr.make_class('C', ['x', 'y', 'z'], slots=True, frozen=True)" "C(1, 2, 3)"
.........................................
Mean +- std dev: 450 ns +- 17 ns

$ pyperf timeit --rigorous -s "import attr; C = attr.make_class('C', ['a', 'b', 'c'], slots=True, frozen=True)" "C(1, 2, 3)"
.........................................
Mean +- std dev: 425 ns +- 16 ns

$ pyperf timeit --rigorous -s "import attr; C = attr.make_class('C', ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'], slots=True, frozen=True)" "C(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)"
.........................................
Mean +- std dev: 1.01 us +- 0.04 us

$ pyperf timeit --rigorous -s "import attr; C = attr.make_class('C', ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'], slots=True, frozen=True)" "C(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)"
.........................................
Mean +- std dev: 1.18 us +- 0.04 us

$ pyperf timeit --rigorous -s "import attr; C = attr.make_class('C', ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'], slots=True, frozen=True)" "C(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)"
.........................................
Mean +- std dev: 941 ns +- 45 ns

Pull Request Check List

• Added tests for changed code.
  Our CI fails if coverage is not 100%.
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