Autotune too slow for backends other than numpy

Investigate why filter tuning can be exceptionally slow for backends other than numpy.
- Focus on matvec first.
- Slowness persists when L1 is used as optimization measure instead of AUC for matvec, investigate potentially slow operations (e.g., indexing?) in that project.

Related tests: None (change backends in the current *compare_filter_tuning.py* in the playground)