It's a Samsung Galaxy S GT i9000 (yes it really is a GTi)
Annoyingly slow,
Anything I can do to get it running at a reasonable speed
It's not loaded with apps,
Example, pressing the back button on an app and you wait about 4 or 5 seconds before anything happens, pressing the button to make a phone call and you wait anywhere between 5 and 10 seconds!
Is it an app that's making it slow or is it just crap?
Apps are
Noom
kindle
Twitter
weather zone
Picasa
eBay
Banking app
Amazon
Data manager
Sd maid
Samsung push
Barcode scanner
Swiss Army knife
Ad service
Reboot phone or factory reset.
This particular phone has a software fault which causes slow I/O performance and lag.
To properly fix it you will need to change the filesystem to Ext4, using for example [url= http://forum.xda-developers.com/showthread.php?t=774500 ]Voodoo Lagfix[/url]
What firmware is on it?
Clear the cache using one of the many cache cleaners available on the Play store.