Few good things come without a price, however, and jQuery is one of them — and while that price may not be directly measured in dollars and cents (jQuery and many of its applications are free); the cost in reduced website performance with some applications could have a dramatically negative impact on your bottom line.
While the near 60k file size of the basic jquery.js file doesn't help initial load times, it's the script's actions that seem to impart the biggest performance hit — with certain meaty applications draining system resources to the point where lesser computers crash.
Proper code writing and basic techniques such as reusing the jQuery object; chaining; using contextual settings; and basic architectural techniques such as searching by ID instead of searching by class name, will all help. Whenever possible, optimize selectors to descend from an id, and don't use too many selectors. Using variables rather than repeatedly selecting the same object and using tag names for class selection also boost the performance of your jQuery applications
Some webmasters may feel that these issues are of no concern to them, as they only use pre-made scripts, rather than writing their own; and thus feel that if the site loads in a fairly reasonable amount of time and seems to be doing what they intend it to do, then everything is ok. This discounts the fact that the visitor's computer could be buckling under the load, and when he goes to play your carefully edited video trailer, his system crashes when the player launches.
You just lost a customer and you don't even know why.
Remember, you as a webmaster or developer are most likely running a far more capable computer system than is your customer, so performance hits that might seem "only slightly troublesome" to you, may be crippling to another user.
For a "seat of the pants" performance test, time the loading of your jQuery enabled site "as is," then flush your cache, disable the jQuery load call and retest the load time. Run several of these tests to average the times if you like, and try it across a variety of computers and platforms; but for some heavy users, the difference in load times will be substantial and obvious. The big question then becomes, "Is the wait worth the benefit?"
This is the real secret to mitigating jQuery resource drains: moderation is the key — Use as few applications as are needed to get the job done right.