Make my website run faster!! (PHP/CMS)

My NEW website is [login to view URL] I am almost finished with the website but I need for it to run more quickly. I have upgraded my server packages and a few other things as advised with the website to speed it up; however, we we began to advertise on social media, problems arise. Before I upgraded from Joomla 1.5 to 2.5 and to Virtuemart 2, the site ended up out right shutting down because it just couldn't handle the traffic. Now, when I do it, the site slows down on loading. I will be fully converting from 1.5 to 2.5 but wanted to make sure that everything is in order before I officially go "live". Once we go like, the Joomla 1.5 will be removed and we will be just operating with "beta". I am familiar with the industry and know more than the average when it comes to website. I just am not a professional and certainly don't have the time to do it myself any longer. Can you take care of this for me in a timely fashion?! Thank you :-)