How to reduce lag

==What causes lag?==
A number of factors contribute to what we all know as 'lag':
* Your computer spec
* Your broadband/internet connection speed and busy-ness
* The internet (in general) busy-ness
* SL busy-ness
* How much is going on in the sim you are in, e.g.
** how many avatars are nearby
** how many textures are visible
** how many scripts are running in the sim
You can't do much about how many people are using the internet or SL at the same time as you, but what you can do is help your computer to work 'smarter'.
==Steps to reduce lag==
===Graphics Preferences===
Starting in 2016 the viewer attempts to set the best quality-performance settings for your hardware. Save the initial settings a 'default' Graphics Preset (icon in the upper left of the viewer).
#Edit Preferences (CTRL+P)
#Select the "Graphics" tab
#If you are running at 'high' or 'ultra' settings, try reducing in steps to 'medium' or 'low'
If this doesn't help enough, step by step try adding each of these changes:
#Reduce the Draw Distance slider to minimum (64m)
#Disable Advanced Lighting Model - This is a drastic hit on quality
#Disable Atmospheric Shaders - This is an extreme hit on quality
#Reduce Avatar Maximum Complexity - Most avatars are <100k
Save the changes as a Graphics Preset
If you want more balanced setting for quality-performance try changing these settings step-by-step. Use the Viewer Statistics panel (Ctrl-Shift-1) to see how each change affects your performance. Open the Preferences Panel and select the Graphics tab. If you have already made changes start at 1 or else 3. If you have already made changes or have the Quality & Speed setting turned down many of these setting will be at their minimum. Changing them will improve quality and some will reduce performance as increased. The changes listed are for improving performance and most degrade quality.
#Enable Atmospheric Shaders
#Enable Advanced Lighting Model
#Click Advanced Settings (lower right button in panel)
#In General
#*Draw Distance is the same as in the previous section. Less is faster
#*Max particle count less is faster when particle are present
#*Post process quality only affects glow. Very little performance gain. Less is faster.
#In the Avatar section
#*Reduce Max # of non-impostors - this is the number of avatars to render. Others are 2D impostors.
#*Reduce other settings in this section. Balance quality to your taste and performance watching Viewer Stats
#In Hardware
#*Set Texture Memory to the max (probably your default). Reduce if you get video errors. 
#*Disable Anisotropic Filtering
#*Change Enable OpenGL Vertex Buffer see if it makes a difference.
#*Only enable Enable Texture Compression if you have a weak video card with limited VRAM.
#*Disable or reduce Antialiasing
#In Mesh
#*Reduce all these settings watching the quality of your render and stats. The amount of performance improvement is highly dependent on scene content.
#**Terrain Mesh Detail affects the ground - very little performance gain
#**Trees affects Linden trees appearance - no performance gain when no trees are around
#**Objects affects every mesh thing in SL - performance gain varies by mesh content in scene, e.g. when building or shopping you might turn up "Objects" and turn everything else down.
#**Flexiprims affects 'tentacle' like prims and clothing prims that bend - performance gain varies by scene content
#Shaders
#*Reducing and disabling these generally improves performance and degrades image quality.
#*Disable Transparent Water 
#*Disable Bump mapping and shiny
#*Disable Local Lights
#In Basic shaders this includes Atmospheric shaders from the previous section. Leave enabled to allow adjusting setting within the group.
#*Leave Avatar Hardware Skinning alone. Enabling provides performance boost. The setting is hardware dependent. The viewer decides at launch if it can be enabled.
#*Disable Avatar cloth - makes mesh clothing ripple in wind if your hardware supports it.
#*Reduce Water Reflections to Minimal
#*Atmospheric shaders leave enabled to adjust settings within the group
#**Sky affects only the sky. Very little performance gain. Less is faster
#**Advanced Lighting Model Leave enabled to adjust settings in group
#***Disable Ambient Occlusion for a small gain
#***Disable Depth of Field
#***Disable Shadows (none) for a huge performance gain
With Graphics Presets you can create settings for various activities. A high quality setting for photos and a high performance setting for combat games. Experiment to find what works best with your hardware. If you really mess up there is RESET button on the Advanced panel that will return all settings to your default.
Reduce any settings you aren't using in you current activity:
#When editing appearance or trying on clothes turn "Avatar" and perhaps "Objects" and "Flexi" up, everything else down
#When terraforming you can turn "Terrain" up
#when exploring, turn "Terrain", "Trees", "Flexi" and "Objects" up
Save the various settings as Graphics Presets
The checkbox for "local lights" will display localised light sources (such as 'spotlights' or face lights). Turning this off may slightly improve lag, but it is a personal preference.
===Other possible steps to reduce lag===
#Remove any render-intensive objects your avatar is wearing (such as sculptie hair, objects with 'bling', very detailed objects) or change them for simpler accessories More information
NOTE: The following struck-out text was written in 2009 or before and is no longer helpful
#Edit Preferences (CTRL+P)
#Select the "Network" tab, then "Clear Cache"
#Re-log (log out and log back in)
Turning off any music or media playing in your SL browser may also help.
===Computer enhancements to improve SL performance/reduce lag===
Some small adjustments or spend on your computer may improve SL performance:
#Turn off any other CPU-intensive packages such as Microsoft Word or PhotoShop
#Turn off any other internet-intensive programs such as internet radio or online games
#Add more RAM - 4Gb is the minimum 8GB is recommended for Window 7, 8, and 10
#*16GB or more if RAMDrives are used for cache.
#Change your graphics card to a better one, 512 MB Minimum
#Change your motherboard and CPU to a newer higher-performance components
#*Faster cores are better than more cores
#*HyperThreading helps
#Faster memory chips for system memory
#Faster Hard Drive or change to a SDD drive for program storage
#*Change to a motherboard that can support the SSD's max transfer rate
#Add a Software RAMDrive that saves contents at power off and restores at power on for caches
#Whitelist your cache directory in your antivirus scanner