FileVault not available on MacBooks? That definitely is not true. I’ve had FileVault on my mid-2011 13” MBA, from at least Mountain Lion up to High Sierra, and have run that without any problems; slowdown or otherwise. And this is on a 1.7GHz Core i5.
Using that and encrypting your disk helps tremendously. Additionally if you’re wanting to dispose of the data on it, instead of trashing the disk, if possible simply pull it, store it, and recycle the Mac. That way you’re not only keeping your data, you’re in physical possession of it.
BL.