Fast enough to jam most amateurs who won't be able to loop it with quality especially if you have the short FH variant to keep them guessing. If you force a push out from your opponent the 3rd ball is relatively easy as it'll be long and slow. If they try a slow low quality loop most likely it's gonna be on the diagonal line and you can simply BH rip it down the line and it's point over. 

The sidetopspin variant is extremely potent against those who like to push. 

Players that aren't at the level to make a strong return on these serves are unlikely to be playing anyone good enough to do the serve in the first place.

That serve should get crushed by any attacking player 1900 or above.

Lol I saw pushblocker using similar long FH serves against 2200+ players and they were eating them for breakfast, lunch and dinner...

It all depends on how good the serving level is relative to the receiving level. Heck even Ma Long uses them a lot. Sun Wen used it against Harimoto so many times and Harimoto didn't manage to get even 30% success rate from my estimation.

Harimoto seemed really unsure of how to deal with it and didn't adjust well in the match blahness mentioned. Match tactics were perfect by Sun Wen that match.

It won't always work against opposition but is a great arrow to have in your quiver. 

Also, he's most certainly not trying. He's just demonstrating the reaction of the serve off his racket

The fact that he is not trying is very important to note.

And the difference in levels is important as well.  The deception can make the receiver make errors. 

And these are side backspin,  not really pure backspin.  With pure backspin, the way the ball slows down is more visible.