If you're coming from Cauldron, you'll notice somewhat of a drop in performance (actual mileage may vary) when using pure forge + FE servers. This is because Cauldron includes Spigot performance patches, which for some reason the Spigot team didn't bring to the Minecraft Forge platform.

• Install Player's [FastCraft] (http://forum.industrial-craft.net/index.php?page=Thread&threadID=10820) (your mileage may vary).
• Ban chunkloaders and whatever you think players can use to abuse server computing resources
• Petition Spigot to support Minecraft Forge (you can try)

I can't give you a good explanation because I don't know how some mods are coded. Poorly written tile entities? Long looping code that runs per tick? Simply too many mods or too many players?

What I can suggest you do, though, is to hook up a profiler to your server (I recommend sk89q's WarmRoast ) and look through the information the profiler spits out, which can be valuable.