Why are the supported opencl related functions slower on gpu than on cpu

Here is my output of clinfo:
Device #0
Name: Mali-G310 r0p0
Type: GPU
Version: OpenCL 3.0 r47p0-01eac0.01561d4.3a706e57d4ed33080af820beda5ab9fe
Global memory size: 15 GB 359 MB 644 kB
Local memory size: 32 kB
Max work group size: 512
Max work item sizes: (512, 512, 512)
When I use OpenCL to run functions such as cvtcolor, its performance is not as good as that of the CPU. Has anyone encountered this problem?
Is it that the OCL module is not compatible with my GPU? Do I need to change any parameters or variables?