The sentence suffers stylistically for several reasons—and the second article shouldn't be there at all.
Any of the following would be more natural:
The more one's effort is sincere, the more one's growth is fast.
The more one's effort is sincere, the faster is one's growth.
The more sincere is one's effort, the faster is one's growth.
(I personally prefer the third sentence, but that's subjective.)
These variations do several things:
- Pronoun consistency is maintained. It's awkward to use one's in the first part and their in the second part. It almost seems as if their is referring to somebody or something else.
- Is is removed from the end of each clause. The normal order of a sentence is Subject-Verb-Object (S-V-O). The first version uses that. While the third version actually uses O-V-S, the fact that the verb is still in the middle means it hasn't changed all that much. (Meanwhile, the second version uses S-V-O in its first clause, but O-V-S in its second clause.)
But using O-S-V, as the original sentence does, is different enough from normal to make it odd—as if Yoda (the character from Star Wars) were speaking. As he might say, "Better it is to do that not not to do that." - Sincerer is avoided. Although it is a technically correct word, and -er is fine in general, it's simply unidiomatic here. It's rarely said—more sincere is more common.
To really paraphrase the sentence, and use a structure borrowed from Spider-Man, it could even be written in the following way:
With greater sincerity comes greater growth.