HFS+ journal on a different volume
A feature originally intended for Server, but possible with non-Server versions of the operating system. Mac OS X Server v10.6: Moving an HFS+ Journal to a different volume discusses using a partition of (for example) an SSD for the journal for some other volume.
… Journaled HFS+ file systems can benefit from placing the journal on either a separate disk, or on a smaller, faster device. …
Apple's published diskutil(8) Mac OS X Manual Page is not yet updated for 10.7 so if you use Lion, pay attention to the local man page — the section for the
moveJournal verb is different.
Whichever manual you follow: if you move a journal, proceed with caution! If the command is not well constructed, the effect can be unexpectedly destructive (I have made such mistakes).
I have not experimented enough to estimate whether moving the journal for a JHFS+ volume could be beneficial to opening poster richardtallent.
External journal on the same physical disk
For some time I had the Apple_Journal type partition for a Core Storage-encrypted Time Machine backup volume on the same disk as that backup volume. After accidentally disconnecting the FireWire cable I had a devil of a job repairing volume structures (details of which are off-topic). I don't know whether having the external journal on the same physical disk was contributory to that devilishness.
If considering an external journal, it may be saner to have the journal on a separate physical disk/device.
Journal for the startup volume
Whilst the manual pages do not explicitly warn against using an external journal for a startup volume, results from my early experiments suggest that doing so may be not a sane move.