slow speed on LAN interface

HI, 

i noticed slow speed on LAN interface, when i open GUI from LAN it's very slow but when i open it from WAN side it's quicker. i checked  speed it's 1Gb/s and on the lan switch it's the same, MTU is 1500 by default, and when i upload  sftp file it's very slow but in download it's quicker,  tried to bypass the firewall (30E) and it's very quick. 

please help

thanks