Quite a number of issues are related to HTTP fetching, including some unexpected ones:
all avatars except your own appear grey
in-world objects remain grey, or they repeatedly go blurry and focus
mesh objects do not rez in
your avatar bakes, then goes blurry, then rebakes, etc
inventory is stubborn to load fully
problems with voice
music/video doesn't work or is intermittent
All of these issues can boil down to one common cause: problems successfully using HTTP to fetch textures and avatar bakes from SL servers.
Things to Try
Please try these things one at a time. If one fixes your problems, stop there, don't continue to the next.
If you are wearing any HUD, including Catwa, WowMeh, Toodledeedoo, The Shops/The Mesh Project/TMP, Maitreya or similar, detach it. These (and a few other high texture HUDs) are known to cause in-world textures to rez then blur repeatedly.
If you don't have the Advanced menu
enabled, enable it with Ctrl-Alt-D. Then, Advanced → Debug Settings
. Type in TextureFetchConcurrency - set the value to 4, then close the Debug Settings window and relog.
Preferences → Graphics -> Hardware Settings
→ Enable Lossy Texture Compression: Enabling this, and then restarting the viewer, will cause the viewer to compress textures while they are in Texture Memory, thus reducing the amount of memory each texture requires. This can help reduce or eliminate the constant re-blurring of textures. The caveat is that the compression is lossy, meaning some parts of the texture are simply lost, so overall texture quality might be reduced and may be noticeable especially along sharp edges.
As noted above, the problems are caused by the HTTP protocol - more specifically, it overwhelms some lower end routers. Many people find that HTTP makes rezzing MUCH faster; others instead, those with lower end routers, wireless ones in particular, have the issues described at the start.
If the fixes above work for you, then great. BUT we strongly recommend looking to upgrade to a better router. In particular, Belkin G series and the LinkSys WRT are two which are best avoided.