monitor IO to tmpfs partition?
tmpfs is not a block device, so ordinary I/O monitoring tools are no good for you.
One way to monitor write speed would be to use pv command. pv, Pipe Viewer allows you to see statistics about current process in situations you normally would be blackboxed, such as during compressing a huge log file or creating a tar ball.
Typical use cases of pv include stuff like this:
pv /path/to/your/log | gzip >/logarchivedir/log.gz
tar cvfz - /your/directory | pv >/outputdir/yourdir.tar.gz
I hope this helps you; you didn't tell us anything detailed.