I run a small private server with never more than 10 users at a time on, but I've discussed this very issue with people, even though I rarely drop below 14-18, and even then it doesn't last or I simply restart the server and it goes away.. but I digress. Common things for people to look at would be:
intricate buildcraft piping - always encourage users to use redpower whenever possible, ESPECIALLY for large, elaborate or item intensive projects. basically stay away from buildcraft except in cases where it actually makes more sense to use it instead of redpower (dealing with liquids and other mods such as railcraft is a great example of this)
improper ic2 wiring - encourage users to paint wires and keep any loops to a minimum, especially "grids" or energy in / out mixing
inefficient use of mods - redpower is powerful, and again with the encouragement.. users should think smarter, collect items in ender chests at the source, and extract them by the stack at the destination to avoid long tubes full of single items.. I don't care how much better redpower is than buildcraft at this, efficiency simply has to be better at helping you save resources. (examples of efficiency include pulling full stacks every 2+ seconds on a timer, instead of 8 sorting machines on .2 sec timers, I'm not sure how much this affects server performance, but I know a certain large server bans people for multiple offenses of breaking the "no timers under 2s" rule, which isn't a problem for me, since my server is so small I can counsel each individual user in smarter implementation
deliberate exploiting / epic fail - there is no two ways about this, either someone is exploiting the sticky piston and sand / gravel duping bug, or exploiting some other mod like computercraft or ee, or just plain fudging it, for lack of a better term, deal with this.
I personally don't see a drop in TPS with 10 or so users, I certainly have at least some of the problems I described above going on, but I don't use nearly as many plugins.. (I use worldguard, worldedit, endreset, choptree2, quickbench, silkspawners, dynmap, permissions, and lagmeter) I DO use a third party server management software called Multicraft, but I am not endorsing it, it kind of sucks. I am using a Phenom II x4 @ 3.1ghz with 3gb of ram dedicated to the server, but my map is only about 400mb..
Good luck!