I love running on varied terrain - grass, edges of fields, tracks and trails. I think it's because having to vary my stride slightly with every step to avoid roots, choose the best landing spot, high step over tangly bits means my feet don't get locked into the monotony of tarmac.
Then again, I am not concerned with speed. A route is a route. You run round it. You might compare time round it one day with time round it another, but why? I know when I've done a good run, or when bits of a run have been good, when I have run freely and recovered well.
At least in as much as it matters to me.
It would be absurd for me to get so concerned about time and personal best to worry about the fact that on grass a training run takes somewhat longer than it should on road.
But I fully respect, indeed admire and am considerably envious of, those for whom this is an important consideration.