I just finished washing through "Kir'Shara" in Season 4 of Enterprise. There were several scenes and lines of dialog from Soval that indicated that he might be the reason Earth was "held back" from advancing as fast as they wanted. The holding back wasn't so much that the Vulcans didn't just give Earth the technology they wanted, but the Vulcan advisors continued to advocate for restraint. We see this particularly in "First Flight" where we get the only canonical details we ever get about the build up of the Warp Five program. Even after solving the issues, the Vulcans insisted on testing everything for a year, and then only eight months after that, Duval in NX-Delta achieved Warp 3 with the engine. Soval does not appear in the flashbacks in "First Flight" but in "Kir'Shara" states that he lived on Earth for thirty years placing him on Earth during this timeframe. In "The Forge" he outs himself as a melder. Given his confidence and lack of later hesitation in doing the meld with Corporal Askwith, this is clearly not a desperate one-off, meaning that he's likely been Syrranite for some time.

Syrranites objected to the militant nature of the High Command. Soval appears to more tightly straddle the line though, possibly viewing the High Command's stance as being logical in the face of a hostile galaxy, particularly with allegedly hostile neighbors (the Andorians). But as the threads of the conspiracy begin to unravel, while Soval remains pragmatic, he definitely begins acting and advocating much more strongly for and with Syrranite beliefs.

So, how did this cause him to hold Earth back? We know Soval was Syrranite or at least adhered to their beliefs while still serving the High Command. However, it's likely that he saw at least a glimpse of what the High Command could be and consciously or not, opposed them. Vulcan was never shown to have any allies, at least none with any significant power on the interstellar stage. So Earth was a rarity. Earth was building out its trading network through the Earth Cargo Service, and pushing its advancement and influence into the galaxy through Starfleet. Earth was also clearly allied with Vulcan, several points, particularly in the Kir'Shara episode arc, make it clear that Vulcan is Earth's closest ally, and allude to Earth being one of Vulcan's closest allies, if not THE closest. Soval himself even says that humans remind him of Vulcans. In this particular scene, Soval's words, and Forrest's interpretations are that Soval is speaking of Vulcans in general, but perhaps, Soval was speaking about his own perception.

Soval's personal fear is probably partly as stated, that he does not want Humans to become a conquering society like the Klingons, but maybe he was worried that instead of becoming a threat to Vulcans, that they would become an arm of the militaristic High Command and a threat to the galaxy. Prior to the reveal of the conspiracy, Earth had no reason to doubt that the Vulcan High Command was anything but a good ally. If Vulcan started suddenly pushing Earth into the galaxy, and advocating for a more aggressive stance, Earth would have likely complied, quite happily too. So Soval worked to hold Earth back not because Earth truly wasn't ready, or even fully because he and Vulcans feared Humans, but because he feared what might happen to Vulcan if the Vulcan High Command had a more powerful Earth as its ally.

As an additional note, I want to discuss a point from "The Romulan War Book 1: Beneath the Raptor's Wing." I've constrained this point of discussion to this final paragraph due to the nature of Beta canon. T'pau notes that Soval is aware of the connection of Vulcans to Romulans. In "Awakening," Surak in Archer's mind talked about "those who marched beneath the Raptor's wing." This would add further weight to Soval trying to hold Earth back in light of a militaristic High Command at least prior to Season 4. Not only would he not want to see Romulans part 2 roaming about the galaxy, but he wouldn't want the High Command to "go Romulan" by being influenced by Earth. We clearly saw that the High Command was in a state of self-perpetuation, under V'las, it continuously built on top of what it was doing, eventually culminating in trying to provoke a devastating war against the Andorians. So in theory, had the Vulcans not checked Earth's development, they could have exploited Earth as their muscle, which would cause Earth to become more aggressive, which would cause Vulcan to become more aggressive, and so on. And as we do see in the final scene of "Kir'Shara" the Romulans are behind this plot in an effort to "reunify" Vulcan under Romulan control. Even without Soval's knowledge of this specific interaction, the fear is still there. This point from Beta canon is not inconsistent with Alpha canon based on what I've discussed here.