What does Trfc do?

Anyone know what Trfc0-3 for DIMM1-4 do? Would messing with these setting improve performance on my ram? My sticks are on the slot 3 and 4. I tried google but no luck.

tRFC (Row Refresh Cycle Time) is an internal timing parameter which determines the amount of cycles to refresh a row on a memory bank. If this is set too short it can cause corruption of data, and if it's set too high it will cause a loss in performance but w/ increased stability. The only way to know for sure whether decreasing the time will improve performance in your particular case, is to either run Memtest86+ and check the memory bandwidth (MB/s) after each subsequent change, or run a memory bench like those included w/ Everest or Sandra.

Thanks redduc900. I just wanted to change DIMM4 to 90ns.
How long do I have to run memtest?