This is a tricky subject. Windows Defender is not the fastest for me, nor the one that uses the least resources. Just use it and observe your task manager, do that for other AVs and soon you will see what I mean. But that is not all.
Panda, for instance, uses very little CPU cycles, which may be good but, aside from the danger of having minimal protection when offline, their online service can be slow too. So it won't be using your CPU to analyse files, but you may have to wait anything from less than a second to maybe 30 or more seconds before a file is considered safe and you can open it. So even though your CPU is mostly free, you still have an annoyig delay for many operations, like I found many games would have much slower loading times, etc. Panda AV Free allows you to reduce the default waiting time until the cloud responds from 30s to 10s (or increase it to 60s for a little more security), but the minimum is still too much for my taste.
Due to the complexity of the issue, this is more of a regularity rally than a F1 grand prix. You have to find something that makes sense to you, and not only in terms of absolute speed. You may find that the fastest engine does not come with the right interface and features for you.
I'm using 360TS with custom settings, which include disabling most of its real-time protection, which I do specifically to free resources. But I'm comfortable with that, this may not be your case. Still, 360TS could be a good solution, especially if you use only the cloud engine or the cloud engine + Avira for real time. I would just recommend that you disable the proactive measures included in 360TS/IS because I've seen them block legitimate stuff before, including Windows Updates. That was with an earlier version, but still I'd be careful. Other than that, it's a solid choice.
But again, like I and other members have stated, you will probably have to find what's best for you by trying. Us malwaretippers may not be able to give a single response that will work for you, but we sure have lots of information to share
EDIT: regarding my recommendation of turning off proactive modules, please only consider this if you are not dealing with a very high risk situation, in which it would be probably better to end up blocking a few legit stuff in exchange for increased safety. 360's heuristic engine is powerful too but also suffers from FPs.
