BUG: ExcelWriter.book --> no member ->Slow Execution Time<-
- [*] I have checked that this issue has not already been reported.

- [*] I have confirmed this bug exists on the latest version of pandas.

- [*] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

Edit: As indicated in https://github.com/pandas-dev/pandas/issues/39181#issuecomment-769507379, the source of the performance degradation was trailing nans at the end of the DataFrame after reading - rhshadrach

#### Code Sample, a copy-pastable example

```python
# Your code here

writer = ExcelWriter(# pylint: disable=abstract-class-instantiated
        path=fileName, engine='xlsxwriter')  
    workbook = writer.book # pylint: disable=no-member
    header_format = workbook.add_format({'bold': True, 'align': 'center',
                                         'fg_color': '#80bfff', 'border': 2,
                                          'font_name': 'Times New Roman', 'font_size': 9})

#### Problem description
the pylint detects that ExcelWriter don't have "book" member, but the code works well just have 
a very bad execution time in comparison with the 1.1.5 Pandas version 

#### Expected Output
Save DataFrames in the same file but in different sheets.
Expected time: 30 sec   (pandas==1.1.5)
Current time: 90 sec    (pandas==1.2.0)

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 3e89b4c4b1580aa890023fc550774e63d499da25
python           : 3.8.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19041
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : Spanish_Mexico.1252

pandas           : 1.2.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 49.2.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : 1.3.7
lxml.etree       : 4.6.2
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : None
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : 2.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.5.4
sqlalchemy       : None
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
