When you want fast USB use UASP usb 3.0 case with an SSD.

Most drive cache data and later write them. it happens since windows 2000 on an acer notebook with pentium 850mhz.

Caja also reports false speeds. Regardless where to

Just wait until it'S done.

also check your kernel settings in regards of usb. I refer to gentoo or arch linux wiki. as those guides will give the hole picture in a much better language.

Also consider smaller files take longer as bigger files

I had last weekend something below 10Mib/seconds becasue i screwed up some alignment or something else. so the usual backup of 30 minutes took over 2 hours.