Bukkit

According to the Bukkit wiki, a CraftBukkit server should have 1GB of RAM per 20 player slots. If you follow that guidance and run a reasonable number of plugins, I guarantee that you will not have a problem with lag or memory usage. I can say this with certainty because I'm running a middle-tier (as in, average pricing) hosted servers with the recommended RAM/players ratio, and I run several additional plugins with no issues. This has been my experience over several months running anywhere from 1 to 4 servers with an active player population.

It is normal for your server to spend more time running Grief Prevention code versus other plugins. After all, GP is watching all your players all the time, and is keeping them all happy while not bothering you. Grief Prevention is extremely efficient at doing its job - performance has been and always will be the most important factor in design and implementation decisions, because the only thing worse than a griefy server is a laggy server that nobody wants to play on in the first place.

Of course any server owner will eventually have a problem if he pushes his hardware too much, for example by running lots of heavyweight plugins. If you do have a problem, the solutions are to either add more RAM/cpu power, re-prioritize your plugins to drop a few you don't desperately need or reduce your max players.

If you have a problem with performance, I need details from you before I can take any action. Consult the manual for your server (CraftBukkit, Spigot, or whatever else you're running) to find out how you can turn on timings/performance measurements. After collecting measurements for a full day, generate a report and share that report with me.