I built an village in The End by transporting two villagers through the portal. I locked them in a 4x2 area surrounded by 92 valid doors. I positioned myself at the top of the wall watching the villagers.

I left the computer for ~3 hours. When I returned the screen was blanked (like normal.) It was slow to resume and minecraft was very laggy. After a while all heart particles that had been generated during my AFK period appeared at once causing the game to reach an almost complete halt.

I logged out to remedy the issue. All subsequent reconnecting attempts failed because the client timed out. The server logs stated:

2012-11-28 17:33:25 [INFO] Banemannen lost connection: disconnect.quitting
java.net.SocketTimeoutException: Read timed out
        at java.net.SocketInputStream.socketRead0(Native Method)
        at java.net.SocketInputStream.read(SocketInputStream.java:150)
        at java.net.SocketInputStream.read(SocketInputStream.java:121)
        at java.net.SocketInputStream.read(SocketInputStream.java:203)
        at java.io.FilterInputStream.read(FilterInputStream.java:83)
        at eg.a(SourceFile:178)
        at cg.i(SourceFile:249)
        at cg.c(SourceFile:15)
        at ch.run(SourceFile:93)
2012-11-28 17:34:27 [INFO] Disconnecting Banemannen [/192.168.1.2:58264]: Took too long to log in

Restarting the server and getting another player to the spot made logging in working again.

Update: It also seems to apply non heart particles like my Iron golem lava trap. It's 3x3x3 where the bottom layer is water, second layer is signs and the third layer is lava. The Iron golems are dropped from above into this pit. I left the game running for a couple of hours and when I came back my iron ingots were displaying 8 and the game moved at something like 0.2 FPS. I struggled to move away 16 blocks (because I thought that to be the border where particles disappear) then I logged logged out and back in. This time relogging worked and I could continue as usual. Note that all this is done on a vanilla SMP server.