Agreed. My machine is 8 yrs. old with a HDD and WD runs very well, but of course the Full scan is very sloooowwww! I never notice when it does an automatic quick scan and I'm surprised when I get the notification. I think it does slow browsing just a wee bit though.It is a well known issue, that Windows Defender can cause slowdowns on some computers. There were many posts about this behavior. But on many computers, WD is OK even with small resources and average CPU. Generally, it is also confirmed on AV Labs performance tests.
Consuming 30% CPU resources is not normal. The Quick or even the Full Scan can usually consume about 30% of CPU resources on computers with CPU like the old Intel i3 processor and 4GB RAM. I noticed a better behavior on Windows 10 ver. 1803, because on earlier versions the Full Scan usually consumed 50%. If WD is not scanning, then there should not be any slowdowns.
In my case, I can see occasional slowdowns during Windows Updates, WD updates, and updates from Microsoft store. In most cases, I can see that Windows Defender CPU consumption is simply 0%.
Anyway, if WD is not working, then the light AV + any default-deny setup with script blocking + document protection will be OK.