The release notes say: “New sub-sampling option: You can now choose to deactivate sub-sampled image previews, which are displayed in the main viewer, when setting your parameters. This will let you view the image at a high resolution while applying corrections.”
I can’t find anything in the Mac version of DPL5 that corresponds to this option, unless it’s the option for ‘high-quality’ previews in the Preferences dialog. But is that new? I’m pretty sure it was in PL4.
Introduced with PL version 5, sub-sampling helps to get a better preview while applying corrections – when your system is fast enough. Otherwise you have the option to switch to the old behaviour.