(tips I’ve compiled from various sources.  some you know.  some you may not.  some are debatable… so please don’t throw virtual tomatoes, just post any corrections/clarifications in your comments!)

Speed up your Client (your computer):

1. Lower your Draw Distance.  I keep mine on 96m or 128m unless I’m doing landscape photography or want to see the bigger picture for a few minutes.
2. Update (or roll back) your video card drivers. New is usually better, but not always.
3. Set Graphics to Low or Medium. Only use High for photography/machinima, etc.
4. Close any unnecessary apps when SL/Open Sims are running.
5. Stop any unneeded Windows services, esp. if you have 1 GB RAM or less. I close Print Services, Windows Time, Windows Search, and a few others on my 8 year old ‘puter.
6. Get more RAM! This probably should be #1, lol.
7. Try running in a smaller window vs. full-screen.  See which your graphics card likes better.
8. Running SL on a lappie? Make sure it is on a flat surface or raised a bit for better cooling.
9. Detach any unneeded jewelry, HUDs, and scripted objects from your Avatar.
10. Lowering or raising your Network Bandwidth settings (under Preferences) sometimes helps, too.  Higher bandwidth is not necessarily better (anything 600 or above should work fine).
11. When all else fails, clear your SL/Open Sim cache and then defrag the hard drive it lives on.  Ener and I like Defraggler instead of the Winblows defragmenter thingie.
12. Don’t fear Snowglobe or alternative viewers. You’ll probably get different frame rates in different viewers. Meerkat and Imprudence work well for me in Reaction Grid, but most peeps say Hippo is best (though the newest version is sometimes buggy for some peeps).  Many SL’ers swear by Emerald, although it (and most other alt viewers) may soon be “illegal” according to SL Terms of Service. Currently, LL only lists 3 approved alt viewers.  Ugh.

Weird tips to reduce Client lag:

1. Mute Blingtards (some swear this really helps lag, I have no idea!)
2. Really stuck in a crowd or classroom? Use Advanced –> Rendering and turn off some stuff.
3. I sometimes use Wireframe view (no textures) or turn off character rendering temporarily if I don’t need to move or see other peeps… and I get a 20 FPS jump!  IM/chat still works.

How to speed up a Sim for EVERYONE:

AVATARS:

Reduce Max Number of Avatars allowed on Sim.  Not always practical/desirable, but it does help.

STUFF:

1. Reduce number of objects in the region. Duh.
2. Using more interior walls and less windows/transparent stuff has same effect (if you can’t see it, your computer doesn’t have to build it and render the textures). Break it up so everything doesn’t compete for simultaneous download and rendering time.
3. Space high prim objects/buildings further apart when possible.
4. Landscaping is nice, but don’t overdo it. Same goes for high-prim, high-texture “realistic” cities, etc.
5. Mega prims, unless used sparingly, can adversely affect the Sim.
6. Don’t overdo it with glow and lighting effects. Not everyone is using a $300 graphics card.

PHYSICS:

1. Making some objects Phantom (so SL or your computer doesn’t have to calculate collisions, etc.) can free up some CPU cycles. Ener already made a post about this, yay!
2. Don’t use physical objects unless necessary. Flexi is usually cool, however (rendered Client side).

SCRIPTS:

1. Reduce the number of simultaneous scripts running.
2. Use mono scripting (this is a subject of some debate, but in general mono is better… esp. for longer scripts).
3. Reduce the number of “listens” in scripts and remove “listens” when no longer needed.
4. Use llSleep() and slower timers to make scripts not run constantly. Do you really need a Sim FPS monitor to update every second? No. Once a minute or longer will do.
5. Small, efficient scripts don’t use lots of resources. Longer scripts — esp. those with lots of functions — are worse offenders.  But, one long efficient script is better than 5 shorter ones that suck but accomplish the same results.
6. Many script functions have a corresponding “off” function that can make things more efficient.
7. Or use llSetScriptState() to turn scripts off/on as needed (such as only enabling a particle script from the main script when it needs to run, then disabling the particle script when its job is done).
8. Once float text is set, you can remove the script that made it. Float text remains :)  Very handy if you have an object that only needs static text above it.  I think the same is true of simple rotations (llTargetOmega)… remove the script, things keep spinning. Correct me if this is no longer true!
9. Disallow user scripts when needed (such as a region that already has lots of scripts running; let’s not bring more!)
10. Peruse a good scripting book, such as the Official Guide to SL/LSL Scripting.

TEXTURES:

1. Avoid use of 1024×1024 when 512×512 images or smaller will do. 1024 should RARELY ever be needed. (Ener can tell you all about this!) I typically use 512×512 for clothing/art, 256×256 for small signs, etc.
2. Textures can affect both the client and the Sim, I think? (Ener, correct me if I’m wrong on this), since textures have to be pulled from the database, then rendered by your machine.
3. Repeat the same texture in different places when possible.
4. Use 128×128 textures for really small/insignificant stuff.
5. Use default SL “library” textures if they will suffice (they may load faster).

Other things that may help:

• Sculpties tend to use less processing/rendering time than high-prim objects. Above a certain number of prims/polygons, sculpties tend to be more efficient… or so I’ve been told.
• Temp-on-rez scripts(when used to defeat prim limits) are to be avoided, unless you use it sparingly, as in an object vendor display. When I do it, I render only temporary+phantom objects, so they poof away in 60 seconds and don’t use any physics time.
• Simple rotations (llTargetOmega) are rendered Client side, so they probably only affect individual users with slow machines, not the Sim in general. But let’s not overdo it with the spinning crapola, okay?