Check your web server logs. You must have enough resources for 40 users AND Google, Bing and Yahoo simultaneously scanning your website.
Your website does not exist to serve customers, it exists to be scanned by search engines so customers can find you. Most people forget to budget enough horsepower for that fact.
American Opulence Analogy
Magento = 45 ft. Beaver Motor Coach
Zen Cart = VW Camper Van
Everyone starts out wanting to power the Beaver Motor Coach with the 4 cylinder VW engine instead of the Detroit Diesel 8V-71 it requires.