investigate and improve startup speed

[davidism]

Use python -X importtime to analyze the start up time based on the time it takes to import modules. Let's see how much we can move into function-level imports to make the initial startup faster. Tools like tuna can analyze the importtime output, and there are also other profilers, including memray, that may show additional places for speedup.

[paxnovem]

I can pick up this issue.