Even with multiple WordPress instances installed, the impact is minimal. For example, I have 30 instances across different machines, with data collected through Webmin Servers Index module for some and locally for others. Even in that setup, the total collection time is about 40 seconds.
But if you have around 5–10 local WordPress sites, it only adds roughly 5 extra seconds overall. Perfectly reasonable and barely noticeable.
For anyone seeing collection.pl problem, there is a new webmin stats sub-command, which will only be available later in the upcoming Webmin 2.600. However, you can apply the patch manually now and use it to get more details about Webmin processes and memory usage.
It explained here: