Is there an existing issue for this?

• I have searched the existing issues for a bug report that matches the one I want to file, without success.

Did you read the documentation and troubleshoot guide?

• I have read the documentation and troubleshoot guide

Environment

- **OS**: void linux
- **Sage Version**: 9.8

Steps To Reproduce

sage: import sage.matrix.matrix_integer_dense_hnf as hnf
sage: hnf.benchmark_hnf([50,100],32)
('sage', 50, 32, 1.9906130000000104),
('sage', 100, 32, 19.642045000000003),

Expected Behavior

hnf is fast

Actual Behavior

hnf is slow

Additional Information

Compare with

sage: hnf.benchmark_magma_hnf([50,100],32)
('magma', 50, 32, 0.02),
('magma', 100, 32, 0.06),

This is taken verbatim from the doctest of sage.matrix.matrix_integer_dense_hnf.benchmark_hnf which thus takes 20+ seconds (on a fast box) which is too long even for "long time" IMO. I'm not sure what this doctest is actually testing here. I want to believe that this used to be much faster, if so this is a speed regression.

I'll make a PR to change the doctest so it doesn't take that much time, while keeping this issue open so the speed regression is not lost.