Thank you for reaching out! Are you noticing slow speeds over WiFi only or are direct ethernet connections to a PC slow as well? What troubleshooting steps have you tried so far?
Tony | Community Mod.
To all of your post, my response is 'Huh?'.
If your area has gotten the latest speed upgrade, there is no 150 anymore. That being said, that router shouldn't have a problem with 150 mbps.
What speeds are you getting? Wired or wireless?
The R6400 should have no issues with 150/300.