I have a simple list with a search.
<ul data-role="listview" data-inset="false" data-filter="true" data-iconpos="right">
My issue is that it loads fine and filters awesomely on my computer, but on my mobile device it makes safari lag really badly(locked up keyboard, horribly slow search). I have an iPhone 4(iOS 5).
The list is pretty massive(5,000 or 6,000) li tags.
Does anyone know a good way to fix this issue?
That's just gonna be too many DOM elements for a 1-GHz processor...
You will need to implement pagination or re-think your data-output to work well on mobile devices. Remember that even really nice mobile devices have the hardware of eight-year-old desktop computers.
Since you have so many list-items, how about doing a back-end search to kick things off? For instance, when the user types a search term, send the term to a server-side script that can then return a list of elements that match, then you could use the jQuery Mobile search filter widget to filter the list further.