It is a very open question, but will try to give you some feedback.
Upgrade. There are many performance improvements in later versions. Since you are using jbpm workflow, use 3.4.d, since 3.4.e introduces the new Activiti engine and is more of a pre-release, and you may not want to try that out for now.
You are not telling anything on your setup. But the more memory and cpu the better. Since we do not now many users you have, it hard to say how much, but a guesstimate would be 6-8G ram.
Separate Alfresco repository and database server, if you have not already done so. In that way, each server gets dedicated hardware for its tasks.
If you are using Alfresco Explorer, consider moving to Alfresco Share. Since Share is only a client, you can run this on a separate server as well, and again gain performance.
I'm not aware of any services that can be turned of, and that would have a noticeable impact on performance. You can turn of file server features like cifs and ftp, but if no one uses them, it won't affect performance in a noticeable way anyway.
