Performance drop on WNDR3700

Dear all,
I installed OpenWRT 18.06 on my Netgear WNDR3700, but I noticed that the download speed (measured with Speedtest) dropped from 241 Mbps to 211 Mbps.

Is this expected? I hoped to get closer to the 290 Mbps of my FFTH connection, and not to further reduce the bandwidth...

Thank you in advance for any suggestion!