What is a good (cheap) way to slow down a fan with a thermistor?
so Idea is to lower its speed by few % only


The first circuit will work with the correct resistance values, but wastes power. A few % will make little difference to the cooling, so remove R2 (the thermistor?) and adjust the resistance of R1 to drop the fan speed by a few percent. So long as the resistance is low enough for the fan to start up it should be fine. You could also try it without the capacitor.

If you need temperature control then combine that with the second circuit by putting the resistor across the FET Drain-Source. This will make the fan run a bit faster when temperature exceeds a threshold. The FET won't dissipate much power so it can be quite small.