I have the 128G for about a month now, and the disk is very slow. Last one I’ve had - Samsung EVO 840 120GB was working much faster even though was older.
It takes a lot, lot of time to install games, aplications, but the worst of it is copying the data - TAKES FOREVER, and speeds are same or worse that on my old HDD.
AHCI is enabled in my motherboard (asus) bios.
SSD is connected to a sata 3 port on the motherboard - I know, it’s no SATA 6Gb/s but 3Gb/s sholudnt impact performance that much - with Samsung EVO 840 I’ve never noticed such issues.