Because it's using feedback, the amount of time that it's going to spend in transition is going to be heavily affected by the value in the right inlet. The lower the value, the more it's lowpassed, meaning that the feedback is greater.

Though you can technically use onepole~ in this way (to filter what's essentially a DC signal), I'd suggest using slide~ or rampsmooth~ (or curve~ if your input is control rate). They work much better, and slide~ is almost obscenely efficient considering that you can decouple attack and decay so nicely.

Thank you very much Peter.

matteo