My inventory is around 150,000 items on my main account, and under 10,000 on another. I have noticed a HUGE problem in V3 and also Kirsten's viewer with my main account. Every time I even *touch* the inventory window, the viewer goes unresponsive for 10-20 seconds. It's too painful to even use it. I tested the same viewers with my newbie account and everything runs smoothly. So I'd say in some situations, yes, a big inventory can cause frequent and unbearable hangs, which some people might refer to as "lag".