Hi Jon,
I'm starting to think this is some sort of IO issue, as you had asserted earlier -- but it's still strange none the less.
I tried a few experiments -- these were conducted on our Mac Pro.
First I selected 10,000 items to be exported, all of the same file format (png images). They were going to be exported into one folder, no directories.
Here are the time stamps per 1,000 items processed.
1,000: 0:06 (6 seconds)
2,000: 0:17 (11 seconds)
3,000: 0:34 (17 seconds)
4,000: 1:01 (27 seconds)
5,000: 1:34 (32 seconds)
6,000: 2:18 (46 seconds)
7,000: 3:12 (54 seconds)
8,000: 4:05 (53 seconds)
9,000: 4:57 (42 seconds)
10,000: 5:50 (53 seconds)
So the next experiment I had was to start exporting them in 5,000 item chunks. I had them originally exporting into individual directories. So chunk 1 went into Directory 1 and chunk 2 into Directory 2 and so forth. All exports were under 2 minutes.
I then had them export into the same directory, here were the results, total time for each export:
1st 5k chunk: 1:47
2nd 5k chunk: 5:47
3rd 5k chunk: 8:24
4th 5k chunk: 12:10
So, for whatever reason, when I export them into the same directory, each progressive one takes longer and longer. Obviously this is arelatively small sample size, but it's still hard to wrap my head around it.