DOC: parallel safe
@datapythonista this may have been why the parallel option wasn't helping you.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=h1) Report
> Merging [#24638](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/510afa70bc3513e64848ad9cbd21c2acbc0f6459?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24638/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24638   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52388    52388           
=======================================
  Hits        48396    48396           
  Misses       3992     3992
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=footer). Last update [510afa7...b2efc64](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=h1) Report
> Merging [#24638](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/510afa70bc3513e64848ad9cbd21c2acbc0f6459?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24638/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24638   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52388    52388           
=======================================
  Hits        48396    48396           
  Misses       3992     3992
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=footer). Last update [510afa7...b2efc64](https://codecov.io/gh/pandas-dev/pandas/pull/24638?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. will merge shortly. thanks! I built the documentation with 4 jobs on master now, and took 5m30s (was 17m30s before). It's working as expected now, thanks!

Not sure if there is something similar for `python setup.py build_ext`. It also ignores the `-j` option for me, and doesn't parallelize.