Speed up GetTerms

[bep]

name                               old time/op    new time/op    delta
TaxonomiesGetTerms/pages_100-10      5.25ms  5%    5.13ms  4%     ~     (p=0.486 n=4+4)
TaxonomiesGetTerms/pages_1000-10     30.1ms  1%    26.8ms  1%  -11.13%  (p=0.029 n=4+4)
TaxonomiesGetTerms/pages_10000-10     1.33s 24%     0.29s  2%  -78.42%  (p=0.029 n=4+4)
TaxonomiesGetTerms/pages_20000-10     5.50s 12%     0.83s 28%  -84.88%  (p=0.029 n=4+4)

name                               old alloc/op   new alloc/op   delta
TaxonomiesGetTerms/pages_100-10      4.08MB  0%    4.06MB  0%   -0.59%  (p=0.029 n=4+4)
TaxonomiesGetTerms/pages_1000-10     25.1MB  0%    24.9MB  0%   -0.87%  (p=0.029 n=4+4)
TaxonomiesGetTerms/pages_10000-10     238MB  2%     233MB  0%   -1.94%  (p=0.029 n=4+4)
TaxonomiesGetTerms/pages_20000-10     469MB  0%     465MB  0%   -1.00%  (p=0.029 n=4+4)

name                               old allocs/op  new allocs/op  delta
TaxonomiesGetTerms/pages_100-10       49.5k  0%     48.9k  0%   -1.17%  (p=0.029 n=4+4)
TaxonomiesGetTerms/pages_1000-10       304k  0%      298k  0%   -1.97%  (p=0.029 n=4+4)
TaxonomiesGetTerms/pages_10000-10     3.02M  7%     2.81M  0%   -7.09%  (p=0.029 n=4+4)
TaxonomiesGetTerms/pages_20000-10     5.77M  1%     5.59M  0%   -3.19%  (p=0.029 n=4+4)

Note that the numbers above represents a full site build, but GetTerms is a big part of the site in question.

Fixes #12610