So, my Dell that I purchased a few years ago came with Windows 10. At the time, I hated it (Windows 10 that is). So, I bought a Windows 7 disc, and have been using that since. Several times, I've tried re-installing 10 again (I made an image on an USB). The link to the image file that I downloaded was given to me from Dell, and matches my P.C.. For some reason, it take 7 hours to install from my USB. Is this normal? So, after all the updates and stuff, it's about a 15 hour procedure. Seems kind of excessive to me...
If it's a storage device with USB 2.0 speed or slower yes that could be the right. Try to get a USB 3.0 stick and try from that. Also make sure you are connecting the USB 3.0 device in to a port supporting this, they will be coloured blue on your Dell.
Sounds more like USB1.1 on HDD fitted laptop.
Unless purposely desired, do not use Dell's image - will be full of bloatware. Instead, get it directly from MS media creation tool
The USB is probably some crappy Chinese variant with read speeds in the KB range
You don't need USB 3 for a fast install - I've installed Windows from USB 2 in like 10-15 minutes if not faster