Osmanhamid, Memory frequency in servers are affected by a number of factors including; Speed of DIMM and CPU as well as memory configuration. R710 server supports:  Let me know if you have any further questions.
Will the Dell Poweredge R710 support Up to 1600MHz Memories? As [Manufacturer Part# : SNP20D6FC/16G | Dell Part#:A6994465] is compatible with R710?
Supported Memory Speeds Poweredge R710