Thus far, one of the key factors in Second Life performance has been the class of server hardware hosting the simulator. There are plenty of other non-server items that might contribute to perceived poor performance, of course, but server class versus server utilization is a pretty consistent factor.
Traditionally, each class of server has a numerical designation and represents a certain basic equipment and operating system specification. In the past, some estate owners have opted not to upgrade to a newer class of hardware in order to avoid higher monthly fees. Now the server class system is coming to an end.