After I travel a long distance, or use Nether portals several times—basically, anything that cause large amounts of chunks to be loaded and unloaded—it causes Minecraft to start lagging, and eat ~80% of my CPU for nothing. Literally nothing: Shift+F3 shows that 70% of time goes into "?" category. What's worse, after that I can't even finish Minecraft properly: "Quit game" closes the window, but javaw.exe doesn't exit, and continues to hog the CPU. Usually I kill it after a couple of minutes, and that doesn't cause problems... until today. Today, that corrupted my world, apparently, because an attempt to load it crashed Minecraft:

---- Minecraft Crash Report ----
// I feel sad now

Time: 10.11.13 19:49
Description: Getting biome

java.lang.NullPointerException: Getting biome
at aoj.a(SourceFile:934)
at afn.a(SourceFile:116)
at afn.a(SourceFile:1047)
at bll.i(SourceFile:1328)
at bll.a(SourceFile:919)
at bll.b(SourceFile:813)
at azd.ad(SourceFile:780)
at azd.e(SourceFile:704)
at net.minecraft.client.main.Main.main(SourceFile:103)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Stacktrace:
at aoj.a(SourceFile:934)

– Coordinates of biome request –
Details:
Location: World: (8,0,8), Chunk: (at 8,0,8 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Stacktrace:
at afn.a(SourceFile:116)
at afn.a(SourceFile:1047)
at bll.i(SourceFile:1328)
at bll.a(SourceFile:919)

– Affected level –
Details:
Level name: MpServer
All players: 1 total; [bje['Joker_vD'/332, l='MpServer', x=8,50, y=66,62, z=8,50]]
Chunk stats: MultiplayerChunkCache: 0, 0
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (188,64,303), Chunk: (at 12,4,15 in 11,18; contains blocks 176,0,288 to 191,255,303), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 0 game time, 0 day time
Level dimension: 0
Level storage version: 0x00000 - Unknown?
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Forced entities: 1 total; [bje['Joker_vD'/332, l='MpServer', x=8,50, y=66,62, z=8,50]]
Retry entities: 0 total; []
Server brand: vanilla
Server type: Integrated singleplayer server
Stacktrace:
at biz.a(SourceFile:284)
at azd.b(SourceFile:1951)
at azd.e(SourceFile:713)
at net.minecraft.client.main.Main.main(SourceFile:103)

– System Details –
Details:
Minecraft Version: 1.7.2
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_21, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 1143131512 bytes (1090 MB) / 1297547264 bytes (1237 MB) up to 1908932608 bytes (1820 MB)
JVM Flags: 3 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx2G -Xms1G
AABB Pool Size: 17427 (975912 bytes; 0 MB) allocated, 17427 (975912 bytes; 0 MB) used
IntCache: cache: 1, tcache: 27, allocated: 12, tallocated: 94
Launched Version: 1.7.2
LWJGL: 2.9.0
OpenGL: GeForce GT 430/PCIe/SSE2 GL version 4.4.0, NVIDIA Corporation
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs: []
Current Language: English (US)
Profiler Position: N/A (disabled)
Vec3 Pool Size: 20 (1120 bytes; 0 MB) allocated, 20 (1120 bytes; 0 MB) used
Anisotropic Filtering: On (8)