TL:DR move closer to the router and see if it works faster
i was getting the same issue, none of my other wireless devices were running lower than their rated speeds, and the old card that the 6235 replaced would work at at least 100Mbps, but after installing the 6235 the WTM quoted speed dropped to 58Mbps, and the usable speed was slightly less, i was at wit's end, until i took my laptop to the basement to recover a server, then i realized it had hopped up to 150Mbps WTM quoted and was almost that when i tested it, i'm not sure why but this card seems to be affected by distance more than other cards, even using the same antennas for bluetooth and wifi as the old RTL8191SE, my $20 ALFA USB wifi card gets full 300Mbps outside of my house as well as various other usb cards(AP is TPlink WA801N 300Mbps 2.4Ghz) but for some reason the intel 6235 is only showing 58mbps on the floor directly above, my best guess is that part of the drivers are not as forgiving with signal quality as off brand cards, or there was a problem with the drivers that made them too strict