Does a MongoDB replica set require at least 2 or 3 members

This is a bit light on details - you don't mention some vital information like what exactly is slow (is this access via a driver for example). Replication adds a little load to the primary, but in general is pretty lightweight.

What size instances are you running, and where (replicating across regions etc.)? You might be saturating your available bandwidth when you add the replication data to the mix. I would only expect that though if you were running a write heavy application or if perhaps you were doing slaveOK reads.

Finally, as a general guide to running MongoDB in AWS, check out this white paper:

http://d36cz9buwru1tt.cloudfront.net/AWS_NoSQL_MongoDB.pdf

And these notes in the official docs:

http://www.mongodb.org/display/DOCS/Amazon+EC2

If you want to keep the data, but start outside a replica set, just restart the mongod process without the --replSet and on a different port. That will give you a standalone mongod.

To be completely sure that the replica set configuration is gone from the instance, make sure that the local.system.replset collection is empty.

Once that is done, and you are happy with your standalone instance, you can then restart with a different --replSet argument and go through the replica set configuration process again:

http://www.mongodb.org/display/DOCS/Replica+Set+Configuration

The other option, as you mention, is to remove all the data files and start completely from scratch.