As the router must be compatible with both it should fall back to what's going to work with all its clients.
300 is only seen under the best of conditions. Folk seem to think it should always work.
Try this, set the router to 150 Mbps and see if it's better.
May I ask why my wireless speed is always 72.2 Mbps when my laptop and netbook operate simultaneously? I have a Toshiba Satellite M840 with Atheros AR9485WB-EG and a HP Mini 110 with AW-NB041H (Ralink RT3090). My modem router is Dlink DSL-2750U. When only Toshiba M840 is switched on, I could get the speed up to 150Mbps. Both of them have a N wifi card, but they can't hit the theoretical speed of 300Mbps.Your help is appreciated. Thanks in advance.