Have you (Blizzard) implemented some sort of new anti-bot technology or something?
Because I noticed a similar problem happened when Jagex over at Runescape decided to add bot detection algorithms to their game.
They actually refunded me 40$ because I wasn't able to play the game anymore.
Anyways I have a great system. and I'm totally up to date.
This system install is less than a week old now. and I don't experience any lag on ANY OTHER GAME.
Diablo 3 is the only game thats lagging / stuttering right now.
I'm very much convinced that you've broken something in your latest patch because like I said.
The game used to play perfectly fine and now I'm not able to play at all.
Anyways, here are my system specs.
Operating System: Windows XP Professional x64 Edition (5.2, Build 3790) Service Pack 2 (3790.srv03_sp2_qfe.120411-0334)
Language: English (Regional Setting: English)
System Manufacturer: MICRO-STAR INTERNATIONAL CO.,LTD
System Model: MS-7576
BIOS: Default System BIOS
Processor: AMD Phenom(tm) II X4 965 Processor (4 CPUs), ~3.4GHz
Memory: 8190MB RAM
Page File: 790MB used, 6960MB available
Windows Dir: C:\WINDOWS
DirectX Version: DirectX 9.0c (4.09.0000.0904)
DX Setup Parameters: Not found
DxDiag Version: 5.03.3790.3959 64bit Unicode
Card name: ATI Radeon HD 5700 Series
Manufacturer: Advanced Micro Devices, Inc.
Chip type: AMD Radeon Graphics Processor (0x68B8)
DAC type: Internal DAC(400MHz)
Device Key: Enum\PCI\VEN_1002&DEV_68B8&SUBSYS_200A1787&REV_00
Display Memory: 1024.0 MB
Current Mode: 1440 x 900 (32 bit) (60Hz)
Monitor: Plug and Play Monitor
Monitor Max Res: 1600,1200
Driver Name: ati2dvag.dll
Driver Version: 6.14.0010.7270 (English)
DDI Version: 9 (or higher)
Driver Attributes: Final Retail