Yah, a very small portion of users will have issues, but so will basically every other AV solution have some type of issue with a percentage of users.
the problem is WD is always running by default so most users won't notice the difference between having an AV or not having one
also, many users tend to compare WD, which has no web filter, and other internet security suites, which has many features + web filter. Then, they state that the suites slow down their browsing/downloading speed => I believe this is so unfair
there is no way Kaspersky (for example) has faster browsing speed than WD. However, Kaspersky can be faster than WD after being tweaked and with web filter disabled (virtually nobody does that)
in fact, I have been installing windows on several machines, mostly on weak laptops + 5400rpm HDD. When I forgot to disable WD after installing windows (to install essential softwares), I always noticed it took significantly more time. Disabling WD reduced the installation time by half or 1/3.
By the way, almost all AVs do slow down our PC noticeably. The difference between PC with or without AV is huge, even with a light AV
That's why there are so many people enjoy using their PC free of any AV