The Tech Report writes ATI's Radeon HD 4000 series graphics cards have known performance issues with OpenCL due to the lack of proper hardware locale support.
In the thread, a developer who identifies himself as Matt Taylor wrote, "We're developing using openCL, and have one dev machine with an NVIDIA GTX 260, and another with an ATI 4870. . . I'm sorry to say we are getting approximately 5x the performance from the NVIDIA card, than from the ATI." Performance is so bad, Taylor adds, that his 2.4GHz Core 2 Quad processor outperforms the Radeon "by a factor of two."
AMD OpenCL Compiler Engineer Micah Villmow responded an hour later with the following:
This is entirely dependent on how you coded the kernel and what OpenCL features you are using. There are known performance issues for HD4XXX series of cards on OpenCL and there is currently no plan to focus exclusively on improving performance for that family. The HD4XXX series was not designed for OpenCL whereas the HD5XXX series was. There will be performance improvements on this series because of improvements in the HD5XXX series, so it will get better, but it is not our focus.