I have not done this upgrade, but I would not expect to see performance drop.
Some things to check and do if missing
Have you set Maximum Memory for SQL Server
Have you changed database compatibility level to 10
Have you re-created statistics for all your upgraded databases
Do you have enough memory on the box. </ul>
If these do not do the job, then you need to look at where the slow performance is happening. There is a ‘Troubleshooting Performance Problems’ download for SQL 2005 that gives excellent advice and is also valid for SQL 2008. I suggest you work through that to find where your problem is.