When it comes to security software and performance in general, you will always get mixed results. A huge part of that issue is due to the infinite number of hardware and software configurations when it comes to Windows. Furthermore, everyone uses their computer for different tasks, all of which may, or may not be impacted by the security programs.
In my experience WD, doesn't impact the performance very much if at all. I have tried many 3rd party AV's, as well as trying it on a number of systems to come to this conclusion. Now that doesn't mean it will be like this for everyone. In my case, I game, surf the web, run VMs etc... I don't compile code, or have tons of programs running in the background etc.. I would say that for the vast majority of use cases WD performs very well. However, there has been mention of it slowing things down if you are compiling code and/or have a folder full of .exe's. I don't fall into this category, so I don't notice this impact.
That being said, the only way to know for sure is to try it out yourself. I know this is not a definitive answer, however as I've said, every single program will have mixed results when performance. So I would honestly try it out with your setup and how you use your computer and go from there. I know myself and many others have zero issues when it comes to WD and performance, but I also know some do have issues, but again that's the same for any program. Tasks such as full scans will always use more resources in general and that's equally true for all programs. I don't run full scans myself, unless I suspect something is wrong. I just let the real-time portion do it's thing.