There's a lot of speculation about the SoC used in Samsung's Galaxy S II,
thankfully through process of elimination and some snooping around we've been able to figure it out.
We know for sure it's not NVIDIA's Tegra 2 or Qualcomm. That leaves Samsung or TI.
A quick look at GLBenchmark2's output gives us the GPU string: ARM Mali 400.
TI's OMAP 4 uses a PowerVR SGX, so it's out of the running. This leaves one and only SoC:
Samsung's own Exynos 4210 (formerly Orion).
Exynos has two ARM Cortex A9 cores running at 1GHz. As a result, general performance
of the Galaxy S II is competitive with phones based on NVIDIA's Tegra 2.
The Galaxy S II runs Android 2.3.1 compared to 2.2.1 used by the Tegra 2 phones,
and as a result has better Javascript performance which we see in some of our benchmarks.

 

Physical Comparison
	Apple iPhone 4	Samsung Galaxy S Fascinate	LG Optimus 2X	Motorola Atrix 4G	Samsung Galaxy S II
Height	115.2 mm (4.5")	106.17 mm (4.18")	123.9 mm (4.87")	117.8mm	125.3mm
Width	58.6 mm (2.31")	63.5 mm (2.5")	63.2 mm (2.48")	63.5mm	66.1mm
Depth	9.3 mm ( 0.37")	9.91 mm (0.39")	10.9 mm (0.43")	10.95mm	8.48mm
Weight	137 g (4.8 oz)	127 grams (4.5 oz)	139.0 grams (4.90 oz)	135.0 grams	116 grams
CPU	Apple A4 @ ~800MHz	1 GHz Samsung Hummingbird	NVIDIA Tegra 2 Dual-Core Cortex-A9 (AP20H) @ 1 GHz	NVIDIA Tegra 2 Dual-Core Cortex-A9 (AP20H) @ 1 GHz	Samsung Exynos 4210 Dual-Core Cortex A9 @ 1GHz
GPU	PowerVR SGX 535	PowerVR SGX 540	ULV GeForce @ 100-300 MHz	ULV GeForce @ 100-300 MHz	ARM Mali-400 MP
RAM	512MB LPDDR1 (?)	512 MB LPDDR1	512 MB LPDDR2 @ 600 MHz data rate	1024 MB LPDDR2 @ 600 MHz data rate	1GB
NAND	16GB or 32GB integrated	2 GB, 16 GB microSD (Class 2)	8 GB integrated (5.51 GB internal SD, 1.12 phone storage), up to 32 microSD	16 GB integrated, up to 32 microSD	16 GB integrated, up to 32 microSD
Camera	5MP with LED Flash + Front Facing Camera	5 MP with auto focus and LED flash	8 MP with autofocus, LED flash, 1080p24 video recording, 1.3 MP front facing	5 MP with autofocus, LED flash, 720p video recording, VGA MP front facing	8 MP with autofocus, LED flash, 1080p video recording, 2MP front facing
Screen	3.5" 640 x 960 LED backlit LCD	4" Super AMOLED 800 x 480	4" IPS LCD 800 x 480	4" PenTile LCD 960 x 540	4.3" Super AMOLED Plus 800x480

The GPU accelerated UI used in Android 2.3.1 makes the Galaxy S II feel a bit faster than the Tegra 2 phones,
however that's not always the case. While web page loading feels comparable between the Atrix 4G
and the Samsung Galaxy S II, Tegra 2 appears to handle flash a bit better than Samsung's Exynos.

This is a pretty significant difference in our Flash benchmark, however it does translate into
a somewhat less smooth experience when scrolling around web pages with Flash.
We managed to run GLBenchmark2 on the Samsung Galaxy S II and compared it to our recently
reviewed/previewed Tegra 2 smartphones.

The Mali-400 MP performs pretty well in GLBenchmark2, however it's still a bit behind NVIDIA's Tegra 2.
Note that the Galaxy S II runs at 800 x 480 so its direct competitor in this case would be the Optimus 2X.
These results don't tell us a lot about the GPU's performance other than the combination of hardware and
drivers isn't quite up to par with what NVIDIA has today - at least under GLBenchmark2.
There's so much that can be done with driver optimizations that it's difficult to draw any meaningful conclusions yet.