PERF: increase the minimum number of elements to use numexpr for ops from 1e4 to 1e6
Closes #40500, see that issue for the analysis of speedup vs size.

Another take of https://github.com/pandas-dev/pandas/pull/40502, but hopefully now without crashing the tests because of increased memory use.
This time the tests are passing! cc @rhshadrach i believe you saw memory issues with the previous PR, can you test this out? @jreback the memory issues were actually present on CI (we just ignored the failing tests when merging ;)), and now not anymore ok great