Both are from the book “Das deutsche Namenslexikon” about German surnames.
They write in the lexicon that the name Spieth is probably from Esslingen. The battle cry of the medieval town Esslingen was “Spuet” in our dialect pronounced like “Speed”, German “Spiet”. “Spuet” meant both: speed and success.
(All famous medieval towns in Europe had their own battle crys, because the population had often to defend their hometown).
Maybe our first ancestor was a very successful and quick medieval defender of Esslingen.
The other theory is, that the name Spieth comes from the German word “spiess” that means a spear for roastings meat or other things. Spiess was written in the early times “spit”, like “Water” became “Wasser” and better “besser” for example.
So the first Spieth was a “Spiess-Schmied”, a “spear smith”.
Later it became fashionable to add the “e” and the “h”. You can watch this in many German surnames. The people wanted to have a long name. That sounded more noble.
Here is a good link for all Spieth’s:
It is suitable to make your own family tree!
You will find all sorts of information when you click on the names…