Damn, a stupid mistake:

it should be

camera.x += (target.x - camera.x) * pow(reductionpersecond,time_step/16)

(with reductionpersecond between 0 and 1)

Now it should work.
pow(reductionpersecond,time_step/16) is the corrected reduction factor; if a frame lasts 0.5 secs the factor will be the square root of reductionpersecond, a second frame of 0.5 secs will have another decrease by the same factor, which means that after two frames (1 second) there will be a total reduction of factor1*factor2 which equals reductionpersecond.