kony wrote:

Placing a cap in parallel with the resistor seems to cure startup
problems.

"Cure"?
To what extent?
I mean, does it actually allow a lower working RPM then or
just offset the overly high resistance?

For fans that will spin at, for example, 5 volts but have startup
problems, the cap acts as a short at startup, allowing the full
12v to reach the fan, then charging to the resistor's dropping voltage.
I normally use a 10uF, 15v electrolytic. [Get the polarity right!]

This is an interesting idea, do you have an example of what
gain there will be for any particular fan?

Not off hand. No gain in operational function. But it sure cures the
"I have to help my fan start" blues.