Older generation ssd isn't fast enough to reach bandwidth limit of sata3 interface, so there wasn't a problem. When ssd becomes faster, a faster interface is needed. That's pcie. Normally pcie based ssd is faster on both reading and writing comparing to sata based ssd. Fusion drive is apple's hybrid drive. It is combination of a small ssd and a large hdd. Frequently accessed files are stored in ssd. ssd also serves as a writing buffer. A very small portion of ssd is used as buffer zone. File is written into buffer zone first, then transported to hdd in background so you won't notice the slow down. A very large file could use up that buffer zone, reducing writing speed to hdd level. Also, if you launch a rarely accessed file, it would be very slow.