This is all fairly obvious, no?

At first, write clean code with functions and don’t obsess over call overhead. Once it works, profile, then optimize where it actually matters.

Premature optimization, etc.

Code length will itself become a problem. The instruction cache is limited in size and often quite small. Bloating instruction counts with lots of duplicated code will eventually have a negative effect on performance.

Ultimately, there's too many factors to predetermine which approach is faster. Write clean code, and let a profiler guide optimizations when needed.

> For functions that can be fast or slow, the decision as to whether to inline or not depends on the input.

This is one area where modern JIT runtimes can dominate static compilations. Dynamic profile guided optimization adjusts things like inlining based upon how the live workload actually performs. You don't need to have any data ahead of time.

There are very few cases where I would trade this sort of long tail optimization capability for something like faster startup. Cold start happens once. If the machine rarely stops, it's not very relevant to the typical operational concerns.