Printing a lot of output can be expensive

[JesseDeLoore]

If you have a lot of warnings in your test suite, because you use, for example, some legacy function of your framework and the deprecation warning is printed (multiple times) for each test PyTest can take a lot of CPU and Memory to process your output.
--ignore-warnings will not help because the warnings are first parsed and then ignored.
The python argument -Wonce will still print all warnings once, so you don't lose track of what you need to upgrade, but not incur as much overhead

Another example of output slowing down your test suite is when you compare large objects (e.g. output files at byte level) in an assert. Pytest will try to find all differences for you and print them. If the file is large it will take considerable time to complete. It might even make your CI runner time out

[zupo]

Could you make a PR for this?