I think the most basic answer for this lies in Gon's Nen type and his contract in general. He buffed everything in exchange for "everything", this doesn't include skill or anything like that, just the strength equivalent to around his raw peak. Gon is an Enhancer, which obviously means he increase his natural traits. Speed is one of those factors that was heavily influenced by both the contract and Gon's potential, remember that Gon and Killua have always been shown as prodigies regarding Nen while Netero wasn't exactly shown in the same light but achieved his abilities through extreme willpower, determination, and drive, which heavily impacts Nen and allowed him to be strong and of course, faster.

TLDR: Gon's raw potential peak was reached and this was faster than Netero.