Many NMs are very capable of being low-manned with the appropriate build, and many kill fast; however, in particular, evasion-tank holding with NIN or THF (without appropriate damage support) and kiting techniques can take forever to bring a NM down.
Unfortunately, this severely delays properly equipped groups, particularly on AF+1 item farming. We are forced to wait while an ultra-slow group takes 15+ minutes to kill a monster, whereas our kills take 3-5 minutes. This is a very unsatisfactory situation, and should be adjusted somehow, though I'm not immediately sure what the best response would be.
Slow killing at the expense of a larger group should be fixed, but not evasion tanking in general.