Last night I was testing out summoning with my Wizard (max Knowledge) and I had a pretty large pack of wolves following me around - While this didnt cause any problems it did bring up a concern. Once enough other people are running around with large packs of creatures (specifically in town or crowded dungeons) what will happen in the lag department? I experienced a wee bit of slowdown and was alone- With a few other players running around with 6-8 wolves what are the chances of lagging out badly? Personally I think this could be a problem. I can see ten or so people running around with max summons and causing chaos in town or in PVP.
I propose a stress test to see how many summons we can feasibly get in a town such as Brimstone and if this is going to be an issue.
Proposed solution: Assuming there IS an issue with too many summonables lagging the game I would suggest limiting the number to around 4 and creating a separate summoning spell which allows you to "enhance" your summons (longer summon times/more HP/more attack power)
I am not sure what the stats are on your PC... we have had some fairly large battles in town and out of town during beta 1 with no real lag issues. However you can always check your fps by typing /fps true if you wish to find out what the slow down is. Some parts of town are slightly slower than others for some pcs it is just a lot for them to draw.
Make sure you are in the local window (not global when issueing the command)