RT-AC87U : slow WIFI -LAN communication

Hello!

I have a problem with my RT-AC87U : slow WIFI -LAN communication.

I did some measurements:
NAS - PC (LAN-LAN, SMB) : 60 - 100 Mb/s
NAS - PHONE1[PHONE2, NOTEBOOK] (LAN-WIFI, SMB): 8-15 Mb/s

But if i measure speed WIFI-INTERNET by something like speedtest.net, i see normal speed 50-60 Mb/s

I use merlin's firmware 384.5.

Any ideas?

Thanks.