What is 了 doing in 这个表快了+time？ I am confused about the 了 in the sentence:

这个表快了五分钟。
Zhèɡe biǎo kuàile wǔ fēnzhōnɡ.
This watch is running five minutes faster.

Clearly there is no change of state. Is it for completion? I don't understand what is completed...
Grammatically, the completion is not only for verbs but also for adjectives.  In your example 快了, 快 is an adj. Another example might be easier to understand.  苹果红了,  apples turned red.  苹果熟了 is a similar example.
Here is the dictionary definition about this usage of 了.

了 / le /
助词
1 
（用在动词或形容词后， 表示动作或变化已经完成）:
read twice;
读了两遍
Apples got red.


苹果红了。

And the official 新华字典 has the similar definition:

了


助词



放在动词或形容词后，表示动作变化已经完成: 买~一本书 | 水位低~一米。