No matter of world settings, it may be flat or normal, the server seems to struggle keeping up. Entities are lagging visually (a shot arrow looks like it bounces back and when the server is catching up, it appears in the right place for example), and redstone is really slow (a fill clock for isn't as fast as in 14w21b for example). Also, the can't keep up message is shown multiple times.

Settings:
VBO: on
Mipmap: off
Graphics: Fancy
Render distance: 10
Max framerate: 120
Vsync: off
Alternate blocks: on

Log:
[15:29:53] [Server thread/INFO]: Starting integrated minecraft server version 14w30c
[15:29:53] [Server thread/INFO]: Generating keypair
[15:29:53] [Server thread/INFO]: Preparing start region for level 0
[15:29:54] [Server thread/INFO]: Changing view distance to 13, from 10
[15:29:54] [Server thread/INFO]: DecentM[local:E:c13a986f] logged in with entity id 113 at (-553.7715211151439, 4.0, -2402.963636236983)
[15:29:54] [Server thread/INFO]: DecentM joined the game
[15:30:00] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4418ms behind, skipping 88 tick(s)
[15:30:41] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 21348ms behind, skipping 426 tick(s)
[15:30:44] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2813ms behind, skipping 56 tick(s)
[15:31:16] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 15975ms behind, skipping 319 tick(s)
[15:31:20] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4380ms behind, skipping 87 tick(s)
[15:31:55] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 14830ms behind, skipping 296 tick(s)
[15:32:03] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4799ms behind, skipping 95 tick(s)
[15:32:20] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5785ms behind, skipping 115 tick(s)
[15:33:01] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 20601ms behind, skipping 412 tick(s)
[15:33:04] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2705ms behind, skipping 54 tick(s)
[15:33:05] [Server thread/INFO]: Saving and pausing game...
[15:33:05] [Server thread/INFO]: Saving chunks for level 'Perftest'/Overworld
[15:33:05] [Server thread/INFO]: Saving chunks for level 'Perftest'/Nether
[15:33:05] [Server thread/INFO]: Saving chunks for level 'Perftest'/The End
[15:33:08] [Server thread/INFO]: Changing view distance to 16, from 13
[15:33:09] [Server thread/INFO]: Saving and pausing game...
[15:33:09] [Server thread/INFO]: Saving chunks for level 'Perftest'/Overworld
[15:33:09] [Server thread/INFO]: Saving chunks for level 'Perftest'/Nether
[15:33:09] [Server thread/INFO]: Saving chunks for level 'Perftest'/The End
[15:33:15] [Server thread/INFO]: Saving and pausing game...
[15:33:15] [Server thread/INFO]: Saving chunks for level 'Perftest'/Overworld
[15:33:15] [Server thread/INFO]: Saving chunks for level 'Perftest'/Nether
[15:33:15] [Server thread/INFO]: Saving chunks for level 'Perftest'/The End
[15:33:26] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5348ms behind, skipping 106 tick(s)
[15:33:26] [Server thread/INFO]: Saving and pausing game...
[15:33:26] [Server thread/INFO]: Saving chunks for level 'Perftest'/Overworld
[15:33:26] [Server thread/INFO]: Saving chunks for level 'Perftest'/Nether
[15:33:26] [Server thread/INFO]: Saving chunks for level 'Perftest'/The End
[15:33:56] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 16453ms behind, skipping 329 tick(s)
[15:34:01] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4965ms behind, skipping 99 tick(s)
[15:34:36] [Server thread/INFO]: Saving and pausing game...
[15:34:36] [Server thread/INFO]: Saving chunks for level 'Perftest'/Overworld
[15:34:36] [Server thread/INFO]: Saving chunks for level 'Perftest'/Nether
[15:34:36] [Server thread/INFO]: Saving chunks for level 'Perftest'/The End
[15:34:36] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 20561ms behind, skipping 411 tick(s)
[15:34:42] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2063ms behind, skipping 41 tick(s)
[15:34:58] [Client thread/INFO]: [CHAT] Saved screenshot as 2014-07-25_15.34.58.png
[15:35:19] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 23180ms behind, skipping 463 tick(s)
[15:35:24] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4515ms behind, skipping 90 tick(s)
[15:35:59] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 17541ms behind, skipping 350 tick(s)
[15:36:03] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3684ms behind, skipping 73 tick(s)
[15:36:33] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 14563ms behind, skipping 291 tick(s)
[15:36:47] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 9690ms behind, skipping 193 tick(s)
[15:37:07] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 11404ms behind, skipping 228 tick(s)
[15:37:21] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 9989ms behind, skipping 199 tick(s)
[15:37:51] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 16078ms behind, skipping 321 tick(s)
[15:37:55] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4017ms behind, skipping 80 tick(s)
[15:38:24] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 16272ms behind, skipping 325 tick(s)
[15:38:27] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3645ms behind, skipping 72 tick(s)
[15:39:02] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 20544ms behind, skipping 410 tick(s)
[15:39:06] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4023ms behind, skipping 80 tick(s)
[15:39:41] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 17591ms behind, skipping 351 tick(s)
[15:39:44] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3147ms behind, skipping 62 tick(s)
[15:40:10] [Server thread/INFO]: Saving and pausing game...
[15:40:10] [Server thread/INFO]: Saving chunks for level 'Perftest'/Overworld
[15:40:10] [Server thread/INFO]: Saving chunks for level 'Perftest'/Nether
[15:40:10] [Server thread/INFO]: Saving chunks for level 'Perftest'/The End
[15:40:10] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2340ms behind, skipping 46 tick(s)
[15:41:03] [Server thread/INFO]: Stopping server
[15:41:03] [Server thread/INFO]: Saving players
[15:41:03] [Server thread/INFO]: Saving worlds
[15:41:03] [Server thread/INFO]: Saving chunks for level 'Perftest'/Overworld
[15:41:03] [Server thread/INFO]: Saving chunks for level 'Perftest'/Nether
[15:41:03] [Server thread/INFO]: Saving chunks for level 'Perftest'/The End