1. It's hard to use them in libraries due to restrictions on how to load them, Opera, Ie 10 and safari 5 require a same origin file, makes it hard to drop a w

1. It's hard to use them in libraries due to restrictions on how to load them, Opera, Ie 10 and safari 5 require a same origin file, makes it hard to drop a worker into a slow part of jQuery. 2. Message passing can be slow and transferring only works for typed arrays which take time to convert to. Unless your calculations are much bigger then your data or converting to typed arrays isn't a bottleneck it may slow you down. 3. OO based inheritance patterns make it very hard to put pieces of your code into the worker, if your slow thing involved a call to jQuery. Ajax you can't call jQuery from the worker because it references the DOM but you can't pull out the Ajax method because it relies on $. extend and others meaning you have to rewrite $. Ajax from scratch (mot too bad in this case) or do major surgery on the library.