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.