Hi Alberto,
Here is a link to an article related to tuning the number of flushers on a D3/Linux 64 bit system using more than 3.5GB of memory.
https://my.rocketsoftware.com/RocketCommunity/s/article/Tuning-the-number-of-flusher-processes-for-64-bit-D3-AIX-and-D3-Linux
In addition to this article, the 'd3 -qf' command is also helpful as it will show how much work each flusher is actually doing. If you have configured your system to use 16 flushers and the 'd3 -qf' command only shows 10 flushers doing any work then it's pretty safe to reduce the number of flushers to 10. The key here is that you want to do this after your system has been running for a while and has gone through some busy cycles so you get an accurate reading before making any change.
About the backup, are you saying that you changed the number of flushers and that impacted the speed of the backup? Or are you asking if having more than one flusher slows down the backup in general?