What performance-degrading issue is the paging file subject to if it's moved to a partition containing data?

I will assume you mean a page file as in a Windows Temporary file used for swap space in the case of low RAM.

Whenever you have the page file on a device that also contains other data that means the device (hard drive, solid state disk, network share) will need to split it's time between sharing data on the swap file and sharing data the computer needs like files or video or what have you.

In order to gain maximum performance from your swap file it's ideal to give it it's own physical disk. Some small SSD's are ideal for this, as are RAMdisks and high speed hard drives of a smaller type. 10,000+ RPM for example.

Obviously this is not very price efficient. It's also been recommended to (if your Windows OS Supports it) split up the page file on multiple hard disks. Keep in mind different partitions are NOT the same as different hard disks.

Hope that helps!