Same here. It's probably a server or host issue. Are you using a dedicated or shared server? Also, if you are using a dedicated server already, then it may be time to try upgrading your server or changing hosts. Compared to you previous site, Magento does use more server processing power, this will translate to more server lag depending on how many people access the site. You could try checking the logs when you get some slowness when browsing the site to see if it correlates with how many people are browsing at that given time. There are plenty of ways to check to see what is causing the lag.