There could be a number of reasons why you are seeing the unload appear to slow down:

• a change in usage pattern on the computer. I.e. the database (or computer) may be used move heavily than when the unload started
• the table(s) that you are unloading when the slowdown occurs could be much larger than the ones unloaded during the initial (faster) period - i.e. larger tables take longer to unload than smaller ones
• the table(s) that you are unloading when the slowdown occurs could be much more fragmented and hence more I/O is required to "jump around" the table in order to unload the data in primary key order
• combination of the two above: the table(s) is really large - larger than what can fit into the cache and the table is fragmented so the server ends up thrashing (i.e. pitching one page out of the cache to read another only to find out that it needs to reread a page that it has pitched earlier)
• change in the disk usage pattern on the computer; other programs (or the database server itself) could be using more I/O for other purposes and hence slowing down the I/O to complete the unload
• an example of increased I/O for other purposes: a checkpoint may have been started during the unload and this will (briefly) stop all other operations from running and then will run for a period of time (in the background) to flush all dirty cache pages from the cache. I.e. an unload which is done during the same time that (a varying amount of) updates are occurring on the database could result in varying unload performance.

You could try using -u switch so that the data is unloaded unordered to see if this improves the unload time. Increasing the cache size (-c switch) may also help.