If you have a slow Sony Xperia Arc, then probably you have Timescape and carrier bloatware that makes your phone slow (e.g. Rogers in Canada). In this tutorial, I will explain the steps that I did to make the phone considerably faster.
Install PC Companion and Update Service
In order to flash a new ROM on your phone, you will need first to install Sony Ericsson's PC Companion as well as Update Service on your Windows computer. This is needed so that the required USB drivers are installed and configured.
After that, download and install Flashtool.
Install Global Generic 4.0.2.A.0.42
Then, you need to get the Global Generic 2.3.4 version, from here. This is build 4.0.2.A.0.42. It will work on your LT15a (American) just as well as the LT15i (International).
Extract the .ftf file that is in the .zip you downloaded and place it under the "firmware" directory under Flashtool.
1. Start Flashtool, and select the firmware you downloaded.
2. Connect the USB cable to the computer, but not to the phone yet.
3. Power off the phone.
4. Press the "Back" button, while you connect the USB cable.
5. Flashtool will transfer the new version to your phone.
Install ADW Launcher
After that, install ADW Launcher and make it your default launcher.
Your phone should be noticeably faster now ...
You will also get some nice features, such as built-in screen capture functionality, camera zoom, 3D camera, ...etc.