I have an Asus AC5300 router running Merlin 380.63_2 firmware. I was using a Asus PCE-AC68 wireless card getting a solid 1.3Gbps (About 10ft apart) with the Asus software that was included and recently decided to upgrade to the PCE-AC88 to get 2.1Gbps. I noticed that the PCE-AC88 doesn't come with any software other then the driver to install and I have been having major speed issues with this card. After doing some configuring with the router I was finally able to get 1.6Gbps to 1.7Gbps but...
AC5300 with PCE-AC88 Speed Issue
AC5300 with PCE-AC88 Speed Issue