In addition to McLion's question, how large are these original photos in pixels or megapixels (width and height) ... do not tell us megabytes.
Export settings are
File Naming: none
File Settings: Jpeg - AdobeRGB (limit file size to 3500k)
Image Sizing: Long Edge 3500px - 300ppi
Meta: Include All
Post: Do Nothing
These are very large images, of course they are going to take longer to export than smaller photos, but you don't really say how long they are taking. Can you give us some estimate of how long it actually takes you to export one image?
What happens if you eliminate the "limit file size to 3500k", does that speed things up?
One image is very quick a matter of seconds. I am exporting 200 images. I appreciate that this is not going to be seconds. It takes about 1 hour. I am in the process of trying different settings / catalogs / user accounts.
18 seconds per image, given the size of the original images, does not seem out of bounds to me. How fast is your computer? Or another way of asking the question, how old is your computer?
Good point! 18s seems ok. I am going to do better timed tests and comparisons between settings. The computer is latest Mac Pro 3.5ghz. The speed is only part of the issue as the interface becomes very sluggish during export almost unusable. I have read that creating multiple threads on quad core machines can speed up exporting. But Its impossible to do this as the interface locks up.
Do you resize for a matter of more manageable files ? If you aren't you could actually export the files full res and then use jpegmini which is what i use in a professional imaging environment - you would have larger images to use but smaller file sizes - however by not pushing LR so hard initially you should find it will export a lot faster
18 mins is around an average kind of time
18s per image seems slow to me, although the unknown quantity is the number and type of edits that have been applied (some of which can have a very significant impact on export speed). On my late 2013 MacBook Pro, exporting 22mp Raw files (5760 x 3840px) takes 2s per image for unedited files, 6.5s per image for averagely-edited files. So 18s may be slow, but again it depends on the edits.
One thing I would suggest is to update to the latest version of Lightroom CC (2015.7)...IIRC export was enhanced significantly when LR6.0 (CC2015.0) was introduced, but that enhancement was removed I think in 6.1/2015.1. However, it was re-instated around 6.5/2015.5 (it parallel processes exports up to around 3 at a time), so you might see some gain if you do the update.