Unlikely though it may seem, the built-in Win Defender that comes with Win 10 and takes over when there are no other anti-virus systems present is really not that bad.
For other lightweight alternatives you might consider Bitdefender, Avast, or Eset's NOD32. Not as sure about the free versions of each, though.
If you require something with greater strength because you keep spending time on or getting redirected to shall we say darker places on the Internet (or someone else using your computer does), browsers keep getting hijacked by malware, or the threat of identity theft is something you are already coping with, then I'm afraid there is almost always going to be a trade-off with the heavier duty products providing stronger degrees of protection vs. using more system resources, throwing false positives, and otherwise getting in the way of what you really want to use your computer for.