Apple has made a significant change to the “Top Charts” section in the iOS App Store just ahead of its Worldwide Developers Conference next week in San Francisco – it’s now only featuring the Top 150 Paid, Free and Top Grossing applications, where before it was featuring the Top 300 on iPhone, and Top 1,500 on iPad. That means that half the applications that previously counted on being ranked here for discovery purposes are gone and have to be found via search or category listings instead.

The change appears to have gone into effect starting around May 23. To be clear, this change is only affecting the mobile version of the iTunes App Store (the App Store app on iOS devices), not the App Store in the iTunes desktop software. On the desktop, the App Store shows the Top 200 apps.

As to why Apple made this change, that’s less certain. Assuming this is not some sort of ongoing bug, it’s reasonable to speculate that this hints at forthcoming changes to the iOS App Store which Apple is planning to announce at WWDC – possibly something that reduces Apple’s reliance on Top Charts as an app-discovery mechanism, and increases its focus on app search.

Already, the company has made several improvements to App Store search, most recently with the introduction of “related search” suggestions that appear after a user types in a keyword or keywords to find an application of interest. For example, a search for “calendar” may offer similar suggestions like “calendar planner” or “daily planner.”

Apple has also begun toying with using app search as a way to promote its own applications, like how a search for “Music” would return a hit for iTunes Radio, or a search for “maps” links to Apple’s Maps before listing third-party app search results. This same feature could one day be used to showcase “sponsored” search results if Apple wanted to go that route.

As for the reduction of the Top Charts, the change is likely to impact the middle-tier app publishers, says Ian Sefferman, CEO at MobileDevHQ, a service that helps developers with their App Store search rankings.

“To keep their download numbers rising as before, they may have to shift to spending more money on paid installs to stay in the ranks, which will drive more demand for app install ads, driving prices even higher than they are currently,” he explains. “I think this will be a short-term impact, though, and ultimately, this will drive more marketers to more profitable and higher quality organic channels such as ASO.”

However, Appsfire CEO Ouriel Ohayon, whose company has an app-tracking application of its own, argues that the change will have little impact on the affected developers. “It just makes it more painful to see the [top] 200,” he says, adding that the ranks which have now gone missing didn’t really help developers with organic discovery and downloads.

“Most users don’t even get to that part of the ranking — except industry watchers,” he says. “If there’s any good reason to remove this part, it’s because it’s not used.”

UPDATE: Indeed, that is the case, as it turns out. We’re hearing that Apple made the changes to the Top Charts largely for performance reasons, because it takes too long to load the Top 300 on iPhone and the Top 1,500 on iPad. Plus, the company also saw that most users would never scroll past the Top 150, as speculated above.