The version Patch 6.4 notes on the League of Legends website detail the usual balance changes in Riot's MOBA, though this time out they have to explain why "some champions like Ryze and Kog'Maw [are] receiving buffs or nerfs despite having just gotten the opposite the patch before." They also note a global nerf on movement speed, and explain their reasoning:

So what snapshots do we have today? We've got heaps of changes to individual marksmen, some nerfs to top-tier pro play outliers, and a holistic reduction of movement speed. Zooming around the rift is cool and all, but the abundance of bonus movement speed is skewing the early game into a sonic-speed arms-race of who can stack it more, faster. We'll continue to monitor the game's mobility creep over the next few patches, but this is our first push toward solving what could become a larger problem.