Since the iPhone 4S debuted in October, Sprint subscribers have been complaining about slow data speeds on the new handset. Several owners have been reporting dismal 3G download speeds of less than 1mbps on America’s third largest wireless network.

Sprint commented on the situation earlier this month, stating that it was taking the issue seriously. And it seems to have made good on its word, as CNET is reporting that the carrier has started rolling out network enhancements to fix the problem…

The company has been bulking up its network over the last few weeks, rolling out improvements to enhance iPhone performance. Sprint says that initial customer response to the improvements seem to be fairly positive, but a few users are still unsatisfied.

CNET shares some post-enhancement customer complaints:

“Some Sprint customers complained that they continue to see inconsistent and, at times, dramatically slow services. “I live in downtown Chicago and experience this constantly ever since I purchased the 4S,” said Gabriel Mayhugh, a musician who works for a nonprofit organization in Chicago. Mayhugh said he doesn’t get much more than 500kbps, with evening speeds dropping down to 200kbps. “I become so frustrated when my friends on Verizon open web pages in less than a quarter of the amount of time it takes me,” he said.”

Other customers are reporting similar issues, with some experiencing 3G download speeds as low as 70kbps. As you can imagine, this would make downloading applications a nightmare, and using Siri (arguably the handset’s biggest feature) nearly impossible.

CNET recently conducted a few tests around New York and found that its Sprint iPhone 4S outperformed Verizon’s model. We, on the other hand, found the Sprint’s version of the handset to be the slowest of the three carriers, so we definitely advise folks to research Sprint’s performance in their area before purchasing.