Most of SL-Residents knows the situation, when you try to rezz an object and it don't appear, you try to walk and just in the second you have moved a few metres forward it throws you backwards to your starting point. Objects around you don't rezz, everything moves slow or in worst case it don't move any more. This situation is called "Lag". "Lag," so says the Second Life Wiki, "is the colloquial name for slow reaction time when using Second Life. It is a symptom with a variety of causes."
One reason can be, that lag is caused by the users computer, with a low-leveled hardware-configuration (less memory, slow harddisk or a slow videocard), or by using an old viewer, or too much processes running due too many opened programs. It can be caused by a slow leased line, a strong firewall with a packet-filtering etc. But lag also can find it's reason on the simulator itself, called "sim". And so it's no wonder, when some residents have lag on a sim, and others have not. If the cause of lag could be compensated by the pc, so the users will not feel any lag.
But: masses of scripts, special objects (i.e. many many mesh-objects), lower ping-times, mega-prims and offsim-objects and collisions (i.e. by scripted running-around animals) can cause lag. And when you have lag on a sim, so it's hard to find the reasons. One tool is the simperformance-collector, which shows in graphs the health of a sim. Three values, the "Dilation Time (DIL)", the "Frames Per Second (FPS)" and the "Pingtime" can give a good overview - next to other tools - of the simhealth. Sure, they are not the "all-healing" solutions for finding lag-causers, it's hard so find the reasons for lag and often it's a long process, taking lots of time.
But it's a tool which helps to see, how real the lag is and how your anti-lag-actions work. Because the SPC works directly on the sim, excluding every PC outworld. If there's lag on the sim, so the results of the SPC are not discussable.
But that's not all.
OMG - It's Laggy!
It's annoying, when you have a laggy sim, you sent a support-ticket to the Lindens and the answer is, that they can't find any reasons for lag on your sim. And really: if THEY have been on your sim to check he performance, all is running fine in this moment. You re-open the ticket and complain again on the lag on your sim. In the worst case you have residents or tenants living on your sim and they already gets angry on the situation. So, how to proof, that your sim is laggy to the Lindens?
The Simperformance-Collector collects the DIL, FPS, the amount of avatars and the Pingtime during 24 hours inworld. The lag is no personal feeling any more, it's a collection of inworld-collected facts. And the graphs shows an undiscussable proof, how the sim works. The DIL and FPS are grabbed directly on the sim all five minutes. The Pingtime is asked by a server in the world wide web, connected to an Gigabit-Backbone. Even if the Pingtime might differ from server to server, the DIL and the FPS can't.
And these graphs are a wonderful proof to show the Lindens: "here is a problem on the SIM!"