Fastest hard drive on the market max burst read speed, about 550 mb/s. Typical hdd throughput, about 120 mb/s sustained. Potentially throttled by the USB port if not C type.
Modern SSD throughput (not industry leading or special); 7,000 mb/s, sustained.
All things are 15-30x slower when moved to a spinning drive; the question is whether the particular game/application is reading a small amount of data and doing a lot of processing (less noticeable) or reading a ton of data and doing little processing (very noticeable).
External SSDs exist and as long as you give them a usb-c port to connect to, can reach significantly better speeds than any hdd - although less than an internal ssd.
The answer is right there in the first line of your post. WoW isn’t meant to be run on a HDD. I switched to SSD 3 years ago because the loading screens were so painful. I can’t imagine what they must be like now.