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.)<br><br>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* 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.

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To really paraphrase the sentence, and use a structure borrowed from [*Spider-Man*][1], it could even be written in the following way:

> With greater sincerity comes greater growth.


  [1]: https://en.wikipedia.org/wiki/With_great_power_comes_great_responsibility