Also, 5 years is a long time in "processor speed world". Here are the hardware specs on my router, which was released about 30 months ago, and is commonly known as the "best cheap router" (~$60):
Yes, five years is a long time in technology. But not so long in business. Manufacturers will minimise their costs and maximise their margins, and will continue to provide the minimum functionality that "ticks the box" for marketing. So add on functionality like USB hard drives typically are not well supported for most routers. Some brands, or specific models of some brands, do work well though. But it is a bit hit and miss, even within a brand. Firmware changes that improve router performance can steal processor time and memory from attached USB drive support, for example.
Besides, your router is two and a half years old. It is also just a BGNA wireless router, not AC, so not even the latest from back then. While it has a reasonable 8MB of flash, it only has 128MB of RAM, which often has a significant impact on attached storage performance. You didn't mention the brand and model, but at $60 is a very cheap router. So it isn't likely to provide lots of support for the attached USB drive. Basically, consumer routers have gotten cheaper while still providing similar functionality to five or more years ago. Only wireless has been improved really.
So, as I suggested before, do some research on the brand and model of your router, particularly on any user forums for it, and see what people are saying about the attached USB storage performance.