The biggest effects on render-time seemed to be heightening the sampling and ray depth amounts in the renderer tab for camera AA, diffuse, specular, and transmission. Render speed was fast at the start, but quickly got really long; Arnold is by far the slowest renderer out of all the renderers. While I didn't ever time it, if I'm correct it took about 17 minutes to render the singular teapot. Imagine rendering an entire scene like that! The same culprits that I think increased render time are what I think made the overall project better, as well as increasing the render size.
While most parts of each render look mostly the same, the only thing I can really say changed was how dark the shadow was. Rendering got somewhat slower on the higher-setting renders, but nothing crazy as the renderer itself isn't very detailed. The only setting that seems to have changed anything besides render size was the different shadow types making the shadows harsher, and that was a light setting, not even a change made in the rendering setup.
I did change the light colour of the backlight for the last two images, so unfortunately that is impacting the outcome, but, like with Quicksilver, the three renders look mostly the same to me. Once again the only thing that seemed to change was image-size output and shadow density, and I really didn't notice anything else. Similarly to Quicksilver again, rendering speed was minimally changed between renders.
Out of all three renderers, and quite expectedly, Arnold created the best result. While it can be incredibly slow, unlike Quicksilver and Scanline, the way it has lighting that works more realistically just makes for a better scene. Scanline and Quicksilver are fast, but create sub-optimal results if you want a good end product, due to their unrealistic lighting; additionally, Quicksilver and Scanline are very easy to setup, as they have few settings and none, that I found, that make major differences.
Unfortunately, Arnold still has them beat. While Arnold has more settings than I know what to do with, and I still really don't know what to do with Quicksilver or Scanline's, the lights that Arnold uses are just far more customizable, and are easier to at that.
Quicksilver and Scanline seem to just take the easy lighting route of just having the light go where the light's going and ending it there, while Arnold actually has items in the scene give reflections and lets light bounce like it really would; thus, Arnold creates far better looking scenes than the other two can.
In all my projects I choose to use Arnold due to its superior look, but it does take more time than the others. It doesn't usually take too long, but if I have a big scene and I need it quick but don't need it to necessarily look immaculate...
I will just lower the settings. Scanline and Quicksilver need entirely different lights than Arnold does, so I would have to replace every single light in the scene, and I don't know of any way to do that that's quicker than just lowering settings in the render setup menu.