SpeedTest low Over Wireless

Hi, I have several clients ( mikrotik sxt ) connected to a sector with a netmetal rb921 firmware 6.30.2 . If I do a bandwidth test from cpe ( using pppoe - client ) to the ip of a server published on speedtest.net get on average 18 to 22 mb/s (good ) . If I repeat the test with a notebook connected via cable to cpe clients get both speedtest.net that bttest of mikrotik 4-5 mb/s

Ethernet and wireless are into bridge interface.

Thanks for reply.