/ip firewall mangle
chain=prerouting action=mark-connection new-connection-mark=speed passthrough=yes protocol=tcp src-address=(your source add.)
content=
www.speedtest.net
chain=prerouting action=mark-packet new-packet-mark=heavy_traffic
passthrough=no connection-mark=speed
and at ip route table use this paket mark (heavy_traffic) and route it to related gateway....