DOC: Improve clarity and fix grammar
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR improves the "Performance considerations" section of the IO tools user guide:
https://pandas.pydata.org/pandas-docs/stable/user_guide/io.html#performance-considerations