Is this external USB3 drive really this slow for my Macbook TimeMachine backup? I got myself this WD 2TB USB3 external harddrive for backups: http://amzn.to/1X8b3a8

I connected it to the USB 3.0 port of my mid 2015 Macbook Pro 15".

When I run Time Machine, it really takes veeeery long for the backup (about 10-20 hours for a 350 GB initial encrypted backup).

So I ran disk speed test with this result. Is this speed normal and does Time Machine backup really take that long?
The speed you see there is not unusual. I have seen various USB3 external harddrives vary from about 60 MB/s to about 120 MB/s. These speeds can be expected from ordinary hard drives - you'll to buy an SSD to get much higher speeds.

However please keep in mind that these speeds are the bulk transfer speed. I.e. if you transfer only one large file without any file system overhead, you would obtain these speeds.

A backup will contain lots of small files which are individually stored. The random access read/write speeds of these portable USB drives are usually much lower than the bulk speed.

Compared to my experience, it is not unusual for the first Time Machine back to take 10 hours.