Highly inconsistent isolate start times

### Problem description

This isn't necessarily a bug report per se, but isolate start times seem slower than previously and much more variable.

What factors affect the isolate start time in Deploy? For example, in the last 20 minutes or so, isolate start times in my global-db-comparison project are varying wildly from blazing fast (144 ms) to very slow (2.2 sec). Can we expect more consistent, faster, times in the future?


### Steps to reproduce

N/A

### Expected behavior

This is more desired than expected and I do not know the limitations you are working under, but ideally:
* Isolates start times do not vary dramatically and are generally consistent
* Isolates start times are less than 200ms

### Environment

_No response_

### Possible solution

_No response_

### Additional context

_No response_

The issue names no repository files, tests, or entry points. Start by reviewing the reported isolate startup timings and the factors that affect Deploy isolate initialization; done means identifying the source of the variability and determining whether the stated consistency and under-200ms goals are achievable.

Written by the indexing model from the issue text.