I am trying to improve the load speed of my Magento store http://www.jinkou.info the load time for the front page on pingdom tools is 13 seconds! It seems that the delay in the pingdom readout is the green section after "connect"
What is likely to be causing this bottleneck?
Magento certainly does like to use the database a lot and every query means a roundtrip to the disc while nothing else is being done. Make sure MySQL has query cache turned on and has lots of memory assigned to it, for large sites several gigabytes are needed. This is why a dedicated server is best.
From tests I find that the biggest CPU cost is running PHP. Even when using an opcode cache it is still an interpreted language and that is expensive. Run PHP as a separate FastCGI process. If following the traditional three-tier approach this would also be a dedicated server.
Only use Memcache if you have more than one PHP server, otherwise just mount var/cache/ as tmpfs.
Lastly as rpSetzer has said - and no doubt others will say - use block caching.
More block caching.
Page caching.