After vacuum application runs very slow ? is this common behavior ?

Our production database size is about 2TB and we had run into issues and Postgres log did recommend running the vacuum in single-user mode. We have successfully completed running a single user mode vacuum without any options. .. services were online after vacuum. Application load time and response times are very poor and don't load sometimes. Is this common behavior after a vacuum? DO we need to consider any post activities or give some time for the database to settle down?

Did full os restart for the DB server?

Thanks 

Sri.