There is a Camera Raw Cache, is that what you mean? If so, you would find it under Preferences->File Handling
Yes I'm aware of the Cache settings but I don't think that is the same thing (could be wrong). As I understand the cache is simply a memory of your recent edits, file loads, edit history etc. The Scratch disk is a portion of the hard disk that Lightroom will use to process edits when the System Ram gets low. ie, Lightroom will perform edits quite quickly using available Ram until the Ram is full, it will then revert to completing the edits using the Scratch Disk on your HDD and that slows the system down. (I think)
There used to be a Dialogue Box with options for setting up the Scratch Disk but I can't seem to find it in LR6.
I don't think there ever has bee an option in Lightroom for scratch disk?
You may be thinking of Photoshop
You could move the RAW Cache to another fast drive and increase it to 25gb