I've been doing some testing to track down the source of some mysterious lag, and it seems one of the culprits is RedPower relays. Any more than one or two in an area, and things get noticeably jerky. A shame, because they're extremely useful.
Relays? I have tested tons (two Stacks in a Relaychain) of them on my very slow Netbook (I also thought it was a Lagsource) and didnt got noticable Lag. But after i built a few Microblocks (including empty Tubes) it got very laggy in my Workshop, due to the Graphiccard, which is not even supported since Minecraftbeta (I have to use the Alphalauncher to start it there).
But in general you should use short Tubenetworks, because the Tubepathfinding could be the Problem. Thats why my Workshops are normally looking like that...