Apparently the lack of hardware knowledge isn't stopping people from posting bullsh*t. Or making cheap shots like Rule of Two did. (who joined 2 days ago and is already an useless forummember, well done, you'll fit right in)
So this is for the people that actually care.
The SATA controller in the X360 is SATA1 and not SATA2. Not that it matters because MS uses SATA1 harddrives anyway. Especially the 20GB harddrive that came with the older X360 models is particularly slow, with a benchmark that's simular to this 20GB Maxtor harddrive:
Now, while the X360 has USB 2.0 it still benefits from new USB 3.0 flashdrives. Because those flashdrives have an onboard controller made for USB 3.0 speeds. That means a very low access time and CPU load. Both are immediately noticeable when they're used with a X360.
Unfortunately a lot of USB 3.0 flashdrives aren't really that fast, for various reasons. Mostly the manufacturer uses a cheaper and therefor slower controller. The Sandisk Cruzer Extreme 16GB has a fast controller so it performs quite good with the X360. It beats the DVD player handsdown and for X360 users that experience pop-ups in GTA V when played from DVD that's all that counts.
For me, with a 5 year old X360 + 20GB harddrive, installing disc 1 on USB and disc 2 on the HDD fixed all the pop-up issues GTA V had. The game loads faster too and the ingame map has barely no load times at all.