Note also that changing these parameters do not impact currently running drainings, we observed that also in the past. In this post some details.
The default values are quite low for a drain to be fast enough (even when draining only one FS). On our instance, we came with these values and they are fitting for normal use (draining one or two FS). Not sure if they could cause an averuse of resources if draining many FS at the same time like you are doing.
drainer.fs.ntx := 50
drainer.node.nfs := 50