Performance: Make `LoadingSpinner` configurable to only show after `Xms`

[umpox]

Description

We render LoadingSpinner all over the application, but in many places it only shows for a few milliseconds before the actual page content is shown.

We should make it configurable to avoid over-showing the spinner, which might harm UX more than it helps in these scenarios

[sourcegraph-bot-2]

Heads up @taylorsperry @jasongornall - the "team/frontend-platform" label was applied to this issue.

[umpox]

@quinnkeast:

i’d actually say almost 300ms is needed before showing a loading spinner (distinct from, like, clicking a button and having its state replaced to reflect loading happening in response)