DPP4 *is* much slower than most other programs (including DPP3) regardless of GPU (more on that in a bit), but you won't feel it much unless you're at higher-than-FHD and/or on a slower-than-top-class CPU. That's because DPP aims for the best quality possible, and it's often possible to get better results than even Adobe RAW/Lightroom if you know how to use it, unless you need "local contrast" and/or "dehaze".
That being said, there are 3 features cases that worsen noticeably (and irritatingly) the already slow workflow:
1. working on 3k or higher resolutions;
2. enabling "Digital Lens Optimizer" (DLO);
3. noise correction.
If you use 2 or more of those, sit down and cry rivers. 🙂 Also, custom tone curve adjustments and color correction, as well as Auto Lighting Optimizer (ALO) may add a bit of extra drag (and, naturally, the more you add, the worse), though these are much less impactful than the aforementioned.
What I do to counter this issue is:
1. do main edits (except sharpness) on a FHD display (if you're on 4k without dual-display, either work with DPP as a quarter-screen window or manually set display resolution);
1.1. if I need to fiddle with Noise Reduction, do it after the main adjustments, and if I really need speed, disable NR altogether when first editing (remember to write down NR settings beforehand) at this phase;
2. once I'm satisfied with everything, and only then, I turn on DLO and fine-tune the other adjustments (including NR), except sharpness;
3. only then I switch back to a higher resolution (i.e. maximizing on the 4k display) for the final kicks. Expect up to 30s between updates on slower CPUs (i5-750, i5-2500, i7-4500u) and 5-15s on faster CPUs (i7-7700HQ) patched against Spectre/Meltdown;
4. only then I adjust sharpness.
The drag is irrestrictive of GPUs, because they're only used for certain preview operations (i.e. when first opening files) that Canon doesn't reveal (if any). Most of the work is still done by the CPU. It doesn't matter which GPU you own (ATI 5770, GTX 960, GTX 1070), the difference is minimal and if anything you'll only see a small spike in GPU activity when initially opening files (and maybe in some other operations I don't remember). Not sure if it's used at all when converting to JPEG.
Actually DPP4.8 is *much* faster than the initial 4.0 releases. I hope DPP5 will improve on this if and when it's released.
Hope this helps.
(edited to add tags, minor corrections for clarity and precision)
