Nothing really comes to mind. Is there any other clustered aspect to your app (e.g. clustered EJBs, some custom clustered service?) If not I'd look at mod_jk config first, but TBH it would be a strange misconfig that would lead to a 3 minute delay in failover.
Yes, All the ejbs are clustered as well as the mdbs.
We also have a singleton mdb.
Does the number of clustered ejbs affect performances ?