I'm running a quad-core 3.0 ghz machine and picked a fairly aggressive starting site (6x6, river, underground river, magma pipe, HFS, flux, no aquifer). (I imagine the quad-core is largely irrelevant, as DF can't take advantage of multiple cores, other than to ensure that DF gets a core to itself.) At 175 dwarves I have significant slowdown - 20-30 fps from the normal 100. I'm thinking of restarting on a smaller site to make pathfinding easier (4x4, underground river, magma pipe, HFS, flux, no aquifer) - I originally wanted two rivers to increase the total flow I could get through projects requiring constant water (waterfalls mostly) but I think I could do fine with just one. Underground to permit towercap farming.
How many dwarves can other folks handle before the slowdown becomes noticeable? What's your machine like, and what's your site like?
On another note, I think part of the problem is that I encircled most of the map in a giant curtain wall, and for some reason dwarves that can't find a booze barrel that isn't already being used are pathing outside the fort entrance and all the way around the map to the back of the fort, outside the curtain wall, to drink where the brook enters my curtain wall, for some reason. I have 'zone-only drinking' enabled and have a water source marked - the part of the brook right next to my main stairwell, and fisherdwarves recognize the zone just perfectly. Any idea why dwarves might not be drinking at my designated drinking area? Yes it's a zone, yes it's enabled for a water source, yes it's accessible, yes 'zone-only drinking' is on.
It's a problem for two reasons - one, I assume finding a path all the way around the map like that is particularly laborious for DF to compute, but also it's essentially the perfect way to guarantee civilians and children find themselves in a goblin ambush far away from military assistance.