Does MySQL Replication hamper the performance of my DB?
There are two major things a Master has to handle because of a Replication Setup

DISK I/O

The Master has to write every completed SQL transaction to the binary logs

NETWORK I/O

When a Slave is connected, the Master must play traffic cop.


Master writes SQL statement to the most recent Binary Log
Master polls for DB Connections emanating from Slaves
For Each Slave DB Connection, the Master does the Following:


Master receives request for the oldest SQL Statement from the Binary Log since the the Slave's last entry in its Relay Logs
Master sends the following to the Slave's IO Thread


Master Log Filename
Master Log Position
SQL Statement at the Master's Log Filename and Position




If a busy Master has Slaves with Seconds_Behind_Master : 0, simply marvelous.

If either of these metrics become noticeable, you may need to do certain things:


Scale up DB Server


Place Binary Logs on a Separate Disk
Faster Disk I/O
RAID10 (rather than RAID5)


Is raid 5 suitable for a mysql installation?
Master - Slave MySQL setup on a VMWare Cloud — Is it needed? (ServerFault)

More RAM

Scale out the Topology


Add More Slaves
Make Each a Slave Master to Other Slaves
Create Distribution Masters



Here are my other posts about setting up replication topologies


Aug 20, 2012 : Best solution for cross-datacenter MySQL master-slave replication
Aug 20, 2012 : How to use MySQL Server 5.6 for GeoIP replication?
Jun 25, 2012 : Mysql database replication on different vlan/subnet/another site
Dec 19, 2011 : Best way to setup master to multi master replication
Dec 17, 2011 : Mysql Master-Master Replication Topologies on &gt;2 machines
Aug 18, 2011 : With MySQL Replication, what level of resilience is possible?
Jun 01, 2011 : What can we do in MySQL 5.0 Replication to address bandwidth concerns?
Mar 29, 2011 : MySQL high availability, failover and replication with Latency