There are people who can feel WD slow, but there are also other people who do not have such problems and had problems with other AVs (also with Avast or Kaspersky). Personal problems with any particular AV cannot be generalized because they can be related to drivers, special configuration, other security software leftovers, etc. For example on my computer, I do not see much difference with most AVs.
Anyway, WD is known to be much slower than some AVs with tasks that require operations on many files (opening folders with many executables, copying, moving, packing, unpacking, making backups or full scans, etc.). Other tasks ((re)booting the computer, launching applications, web browsing, document editing, watching videos, playing music, etc.) are very fast.