Why does my gas pedal feel heavier?

I have a 1999 honda civic si and i noticed sometimes when i'm driving it on some days that my gas pedal feels a bit heavier and has a little resistance and my car does not accelerate quickly like it used to when this happens. What may be the cause?

Cables could be dirty. Throttle plate could be dirty.

The throttle cable might need adjusting or its mounting bracket is loose. The cable may need a replacement if wore out or like Ron A has said: Clean the throttle plate.

I would give the throttle cable a complete inspection from the pedal connection to the TB connection. Cable is cheap and easy to replace. HTH