It doesn't run slower, In my experience it just can't load quick enough when reading both discs from the hard drive, I found that especially when driving at speed certain posts and stuff would just appear very late on, i.e just as you hit them and stuff like that.
I ended up deleting the second disc off my internal hard drive, and installed it onto an external drive and it works much better.
Basically the "bus" for the usb and dvd drive are seperate to the HDD system bus, so if both discs are being loaded from the HDD it struggles with the bandwidth available to load everything fast enough. So playing disc 2 from dvd drive or usb drive solves the issue.
- Get_Head_Ed likes this