Run by line is very slow when stepping through code in other cells/files/libraries

[roblourens]

I am testing with this notebook:

https://github.com/fastai/fastbook/blob/master/08_collab.ipynb

• Find the cell with the code learn.fit_one_cycle(5, 5e-3)
• Start a RBL session
• While RBL is auto-stepping through code that we don't care about, the code runs very slowly. We expect this when debugging, but we also trigger expensive variables requests on every step. We should only do that when we will actually stop

[rzhao271]

RBL still runs much slower than no RBL. Whereas with no RBL, the progressbar on that cell seems to gain over 10% per second, with RBL, the progressbar seems to gain around 1% per second. @roblourens is this approximately what you were seeing?

[roblourens]

It will always be a little slower due to running in debug mode. There is also a little extra optimization we can do. I think @DavidKutu and I talked about this last week. Opened #7365