Which OpenCL APIs load kernels to FPGA?

You should read the Intel FPGA SDK for OpenCL Programming Guide and Intel FPGA SDK for OpenCL Best Practices Guide. Some basic information about the way the hardware is created and basic optimization techniques and their effects are detailed there. Those guides also detail the reports and how they should be interpreted by the user. At the end of the day you will have to run a lot of experiments yourself to be able to better understand the effect of different optimizations and code modifications.