There is a lot of this and that about lag and there is many factors involved.
A sim people come and leave all the time can have good numbers but just the fact they come and go make lag.
That is because every one has to update that avatar with all scripts,hud's and textures.
Using max texture size on all fancy stuff also slow down. Linden recommend max 1024 pixels but Idle is 512.
I have checked a few things to look at:
Time Dilation: when that go down the simFPS also go down to compensate so sim can run smooth. Idle is 45
Active Scripts: There is no Idle , but lets say under 10000 for a sim total. Higher makes lag . Seen 25000 and i had to leave, could not move.
Script run: Idle is 100% but there is no problem down to 50% . At 20-30% you get trouble with hud's, rez things and so on.
Total Frame time: 22 is idle and under. You will se at most have 22,200 or like that. Problem starts if it get higher and stay so.
Spare Time: As it says, the time over to run scripts. Constant on zero is a problem. Idle is any number over zero.
Hope this can help you
Statistic bar on Firestorm is CTRL+SHIFT+1