Minecraft bad fps on my server?

After spawning too many skeletons with my skeleton spawner on my minecraft server, my client gets terrible fps whenever i join it, but i'm not getting bad fps on other servers!

I get up to 50 fps on pvp and large survival servers, but when i join my server (running on localhost for me and my friends to join) I'm getting massive lag.

Here is the kind of lag i'm getting:
- Large lag spikes when simply looking around
- Walking 5 blocks will nearly make my client crash
- fighting monsters is pretty much impossible
- pausing the game / operating chat takes forever for my game to respond…

I have already tried updating java, restarting my computer, opening minecraft, completely deleting my minecraft version and loading a fresh version, increasing the allocated ram.

Please help me, this is extremely annoying, and i have never had this kind of lag. Any thoughts on what's going on?

Give your server more ram. If you have like 16gb of ram i would give it 10gbs