Magento 1.x has always had a somewhat deserved reputation for providing poor customer facing performance. Let’s put it this way, it is at least hard to get maximum performance. However, with experience and a few practical steps it is possible to get Magento 1.x to provide pretty good performance characteristics even at large scale. The root of the performance difficulties is in the platform’s focus on flexibility; it does indeed come with a price. However, flexibility is probably the single feature that has catapulted Magento to its rightful position as the global leader in ecommerce platforms with 26.3%* of the market share.
Magento 2, scheduled for general availability release later this year, is set to change that by improving the performance characteristics of the platform whilst maintaining the flexibility of the original product.