Ah, the new HDR is certainly shaking things up!
After a little testing, I have to say it is acting as advertised - the initial calculation takes time, but after that changes are nearly immediate. And remember on old HDR certain changes would also require a fairly hefty recalc.
We've become a little bit spoiled by ST-GPU methinks. Remember beforehand, when it was almost mandatory to use things like a preview box in some modules?
Brendan I believe you desktop might be okay, but the laptop could be lacking the oomph to keep up with ST as it advances. I understand though, just a couple weeks ago I was trying to extend the life of my Q9550 desktop. Well, this new HDR would have been the nail in the coffin I think, if I hadn't obtained a replacement computer.
From what I can tell so far, new HDR is very CPU dependent, and even though monitors do show GPU being used, it's not maxed out like the CPU's are. I ran some test data at it on two machines, image was 2380x1668, so a bit on the low side of the resolutions I've been processing at lately (I like to maintain a certain amount in order to avoid squaring off stars and to give SVD room to work):
On my laptop (i7-8550U, 32GB, 940MX), initial HDR took 4:46, but a context level of 25 instead of 50 only took 19.8 seconds. A change to a context of 75 took 2:21 just to get to the second little bar in the clock (so that means go mow the lawn and come back). All cores/threads were maxed out, and were being kept to a turbo speed of about 2.8Ghz. The 8550U is supposed to max turbo at about 4.0, and in certain modules it does (SVD), but not the way HDR is using it.
On my desktop (i7-6700, 32GB, GTX 745 OEM), initial HDR took 3:29, context level 25 took 17.7 seconds, and a change to context 75 took 1:39 to move one clock bar (mow half the lawn!). However, this CPU was able to run all the cores full out at 3.8GHz, which I think is the difference here.
Now, even though the GTX 745 is nothing to write home about, I used MSI Afterburner to OC it by 135Mhz on the core clock and 200Mhz on the memory clock. This does increase my benchmark by a noticeable amount on userbenchmark. However, the GPU overclock made no difference at all to my HDR times.
But, using different dataset at 2920x1945 resolution, this mild gpu overclock did shorten my SuperStructure wait time by nearly 10 seconds, from 1:43 to 1:34. Other modules, which generally don't take as long, like AutoDev, Wipe, and Denoise, were also sped up by a small amount. So it is something worth considering. MSI Afterburner is also free and is not limited to only MSI's own gpu's. Just do some Googling to find out what people have found for safe OC's of your particular gpu.
And although Ivo already noted the ancient nature of the Fermi gpu architecture, I am still of a mind to put my old N460GTX Cyclone into this tower, since it is faster from the get go than the GTX 745, and can also be seriously overclocked. I already tossed a new 700W PSU into this thing anyway, might as well use some of that capacity!
Nonetheless, from this initial testing, it seems to me that you have to look to your CPU regarding new HDR and how long it will take.
I still need to run old and new HDR side-by-side to see how the new settings and controls compare, and if there is anything we are now "missing out on" such that we would want to increase the context level and eat the longer wait times.