The data=writeback mount option deserves to be tried, in order to prevent journaling of the file system. This should be done only during the deletion time, there is a risk however if the server is being shutdown or rebooted during the delete operation.

According to this page,

Some applications show very significant speed improvement when it is used. For example, speed improvements can be seen (...) when applications create and delete large volumes of small files.

The option is set either in fstab or during the mount operation, replacing data=ordered with data=writeback. The file system containing the files to be deleted has to be remounted. //

He could also increase the time from the commit option: "This default value (or any low value) will hurt performance, but it's good for data-safety. Setting it to 0 will have the same effect as leaving it at the default (5 seconds). Setting it to very large values will improve performance". –
Cristian Ciupitu Commented Sep 26, 2010 at 19:14

Lastly, FYI, not mentioned in that link is that fact that data=writeback can be a huge security hole, since data pointed to by a given entry may not have the data that was written there by the app, meaning that a crash could result in the old, possibly-sensitive/private data being exposed. Not a concern here, since we're only turning it on temporarily, but I wanted to alert everyone to that caveat in case either you or others who run across that suggestion weren't aware. –
BMDan Commented Sep 27, 2010 at 1:23