Investigate the potential for reduced compile times in `metrics.py`

### What's the new feature?

See the discussion here: https://github.com/gchq/coreax/pull/661#discussion_r1642508262

### What value does this add?

May reduce compilation time and hence time to first result, particularly helpful for small problems.

### Is there an alternative you've considered?

_No response_

### Additional context

_No response_

Start by reading the discussion in pull request 661, then inspect metrics.py to understand the compilation path under consideration. Measure the relevant compile times before and after any investigated change; the work is done when it reaches a clear, evidence-based conclusion about whether reduced compilation time is worthwhile.

Written by the indexing model from the issue text.