Apple's A4 processors in their iOS devices are powerful. They are able to outperform in most cases other mobile chipsets. The man responsible largely for that feat, Jim Kellar, is leaving to AMD.
According to 9to5mac, Keller was an employee at PA Semi when the company was acquired by Apple, and while at Apple, was listed as "A4/A5 Designer & K8 Lead Architect." Keller was hired by and will work under Mark Papermaster, a former iPhone employee who was dismissed after the whole Antennagate debacle in 2010.