pg_dump with low priority?

We have a huge database which must be backed up every day with pg_dump. The problem is, it takes around half an hour to produce the dump file, and all other processes on the same box are starved for cycles (presumably due to I/O) during the dump. It's not just an inconvenience, it's now evolved into a serious problem that needs to be addressed.

Is there any mechanism for running pg_dump with a lower priority? I don't mind if the backup takes two hours instead of half an hour, as long as other processes were getting their fair share of cycles.

Thank you for any advice,

Bryan