mobile

Hello. Recently I got my hands on this device. Never had experience with flashing Roms/kernels but did my research and got familiar enough to explain the problem.

The phone is very slow and unresponsive. I have checked and it is rooted. So I decided to flash new ROM to it, CM11 ROM + OnePlus One Conversion Kit for Nexus S. However, there is a warning:

"YOU HAVE TO USE CWM Recovery 6.0.4.3 or HIGHER otherwise you will fail during install and other problems will occur!!! You can easily update to latest CWM Recovery by using ROM Manager from Play Store."

So I downloaded ROM Manager to update it and that's where the problems started that I do not know how to deal with, and I don't want to risk bricking. ROM Manager installs fine, but when entering gives following message from first image.
[IMG="Nexus S Album"]imgur.com/a/uBR22#5[/IMG]

So when I tried to ignore that and update it anyway, got a message with error, which resulted in red triangle when booting into recovery. After a while system loads up as usual, phone is working but painfully slow. So if anyone is kind enough to help me restore it to working condition? What to do?