More linear algebra speed issues

[pzinn]

I keep finding new issues with doing linear algebra in M2 (I know this isn't M2's specialty but still...), see e.g. #2718.

I have this sample 461x149 QQ-valued matrix m which I'm not going to reproduce here.

(1) Asking for kernel m takes 16.7s on my desktop.

(2) Asking for kernel promote(m,QQ[]) takes 2.2s. (?!?)

(3) Asking for reducedRowEchelonForm m takes 0.04s (as should be).

Clearly (1) shouldn't take more time than (3).