Client-side performance is just as important as server-side performance, but it's even easier to negatively impact client-side performance, since any server lookups have the potential to lock up the user's browser for a couple of seconds (a lifetime to an end-user!). Even scripted actions which you might not think would result in server lookups, such as updating a reference field value, can result in a synchronous server lookup, which can negatively impact a user's browser performance.
Note
As we learned earlier in this chapter, specifying a third argument (the display value of the record) when setting a reference field, prevents the server lookup, improving performance!
It's important to understand which actions may result in performance-impacting server lookups. It's equally important to understand how (and when) to mitigate that performance impact using asynchronous callback functions, or by combining lookups into one big request and providing all of the information...