I know many readers already know this, but for the benefit of the OP...
It is important to understand there are two totally separate icing issues under discussion and they effect performance very differently
1 - Airframe icing collected inflight- Usually involves only the leading edge of the airfoil and windshields
- Increases stall speed due to extra weight
- Increases/changes stall characteristics due to changing airfoil shape
- May result in an airfoil that is 'unflyable' if you make a configuration change (like extending flaps)
2 - Frost collected when parked.- Usually involves the whole upper surface of the airfoils
- Acts by increasing friction (which contrary to an earlier post is actually true). A small degree of roughness on the wing surface can substantially slow the circulation of air above the wing.
- Has negligable effect on weight
- Can have a dramatic effect on lift due to the reduced circulation discussed above (which is the fundamental physical aspect of lift)
1 - Can be addressed with deicing/anti-icing kit, descent into warmer air.
2 - Can only be addressed with preflight clearing of ice.
The way 2 seems to kill people is, you rotate slightly above normal, climb in ground effect, stop climbing due to reduced circulation, pull pack to increase climb rate, Stall/Spin into the ground.
This does seem to be more common on modern airfoils and even two turbines pushing you sometimes results in the above.