Canon's autofocus algorithms and implementation details aren't public (just ask Sigma and other third party lens manufacturers) so I doubt there's a definitive answer, but I'd be surprised if the DSP in a DSLR contributed to AF performance in a meaningful way if you talking about a Rebel's One-Shot AF mode.
Here's my thinking:
First, Lensrentals has a quick primer on autofocus that's worth a read: https://wordpress.lensrentals.com/blog/2010/07/how-autofocus-often-works/
The lesson here is that phase detect AF (like the SL2's) is based on a relatively small number of inputs. The lens tells the camera's AF its focal length, aperture and current focus point. The phase detect chip tells the camera's AF system how out of phase the auto focus points are. The camera then tells the lens, "I think you need to move focus x distance towards infinity" and the lens drives focus towards that point.
In one-shot mode, focus isn't going to be checked until the lens is at the desired location. In the world of electronics, it takes a long time for that movement to happen--far longer than the original calculation. And that calculation—"how far do I tell the lens move"—isn't nearly as complicated as, say, de-bayering 24 million pixels and then applying JPEG compression, which is something that an SL2 can do multiple times a second.
In short, the processor's speed is likely a pretty small contributor.
So, where does that all go out the window? I'd say the DSP is going to have a more meaningful contribution when:
- Using live view, since the 24mp imaging sensor is used for AF
- On a camera that uses the metering sensor to select an auto focus point
- Evaluating which of a large number (e.g. 50+) auto focus points to select
- Potentially, in your canon's "AI servo" mode, which continually tracks focus. On the 7,5, and 1 series canons the AF menu is blisteringly complex but those cameras also have a lot more feeding into the AF system such as iTR.
Does a 1DX blow a rebel out of the water because it's got a better DIGIC in it, or because it has a more expensive AF sensor, more AF points and an RGB metering sensor? Hard to tease out. For instance, one reason that the 1D-series focuses so well is simply because it has larger batteries that give the lens' motors more voltage so they can move the glass in the lens faster.
Anyways, the bottom line is the SL2 (like any DSLR's) AF system is a well-calibrated modern marvel of complexity but I wouldn't expect the camera to perform any differently in terms of AF performance vs your T3. Take a trip down to your local camera shop to try it out for yourself.