`import/namespace` is very slow

In our medium-sized mixed JS/TS codebase, specifically disabling import/namespace significantly reduced our lint times. I haven't found this mentioned by anyone else, so I wanted to share the finding, because it was very surprising.

Our prior configuration ran like this:

Rule                              | Time (ms) | Relative
:---------------------------------|----------:|--------:
import/namespace                  |  5762.140 |    64.7%
import/no-relative-packages       |   673.212 |     7.6%
import/named                      |   471.116 |     5.3%
import/no-extraneous-dependencies |   317.690 |     3.6%
import/extensions                 |   254.717 |     2.9%
Done in 14.53s.

I always assumed this was just the "first rule tax" while the export map is created, but adding "import/namespace": "off" did this for us:

Rule                              | Time (ms) | Relative
:---------------------------------|----------:|--------:
import/named                      |  1459.028 |    33.6%
import/no-relative-packages       |   577.911 |    13.3%
import/no-duplicates              |   408.152 |     9.4%
import/extensions                 |   401.591 |     9.2%
import/no-extraneous-dependencies |   276.880 |     6.4%
Done in 9.44s.

Given we already use "import/no-namespace": "error", this rule wasn't doing anything for us anyway. With one line we've managed to significantly improve IDE responsiveness.

The relevant entry point is the import/namespace rule. First reproduce the reported lint timings on a medium-sized mixed JS/TS codebase and compare runs with the rule enabled and disabled. Done means identifying and reducing the rule's unnecessary cost without changing its namespace-validation behavior.