IR can compute the GI part in fraction of time and reuse it for multiple frames. That's two domains where it saves the time.
Example: 1 Frame (BF/LC or PT/UHD, doesn't matter) = 2 min for UHD + 2 hours for fullHD clean interior
1 Frame (IR/LC) = 2 minutes for LC, 10 minutes for detailed IR, and and 20 minutes for frame.
So for still image, we get roughly 4 times faster image at best, at worst it can be roughly twice only. It's almost always faster, because since interpolated GI is always smooth (IR cannot be noisy), there is much less noise to deal with after. Specular surfaces render much faster.
But the true time savings come to simple flythrough animation, where single IR map, can be reused across multiple frames, at slow speeds, even 1 IR map per 30 frames.
So 1 minute of animation (BF/LC or PT/UHD) is 1800 (frames) x 2 hours = 3600 render hours
1 minute of animation (IR+ LC) = 180 (every 10nth frame to be safe for every static scenario)x10min + 1800x20 = 630 render hours
The time saving are even better. The whole setup is more complicate, and issue-prone,...but ultimately, we are talking thousands of euros in computation times difference.