XPages are using Java in the background, and the amount of memory Java can consume in the Domino server is limited by the HTTPJavaMaxHeapSize notes.ini parameter.
Default ist 256M.

Recommondation for 32bit systems is 512M.
Set on the server console via

set configuration HTTPJavaMaxHeapSize=512M

Storing large amount of data in a scoped variable

Prefer java objects instead of javascript objects, for example use a java.lang.hashMap instead of a JavaScript array.
Doesn't matter for small amounts of data, only if you want to store many values.

Use partial refresh

If you change something on your page, use a partial refresh instead of a full refresh since partial refreshes only need to process a subset of the JSF tree and are therefore faster.

JavaScript variables

In 8.5.0 think about setting your variables to null when you don't need them anymore.
It is planned in 8.5.1 to have a setting so that all variables are cleared automatically after the lifecycle of a request.

General tips

Blog post of Mark Gargan: http://xpagesblog.com/xpages-blog/2009/8/10/writing-efficient-scalable-xpages.html

Synchronized functions and caching

Matt White wrote about an interesting technique to optimize performance for frequently used functions, see the full post here: http://mattwhite.me/blog/2009/9/14/on-synchronization-in-xpages.html

In a nutshell: you can mark a block of code as 'synchronized' which means that it is not allowed to run concurrently when mutiple clients are doing the same.
A synchronized function is ideal to compute expensive stuff and cache it.

Here an example an example function which computes something expensive and caches the result. If the result is already in the cache, it is returned immediately.
A cached result expires after some time.

function getExpensiveResult(){
  synchronized(applicationScope) {
    if(isCacheInvalid("example_key", 600)) {
       result = // do expensive computation of something, like iterating through a view or so       
       applicationScope.put("example_key", result);
  } 
  }
  return applicationScope.get("example_key");
}

/**
A generic caching mechanism for each key will check to see if it is 'n' seconds
since it was last updated. Use for things that change relatively infrequently  
*/
function isCacheInvalid(key, cacheInterval){
var currentTime = new Date().getTime();
if (!applicationScope.containsKey(key + "_time")){
applicationScope.put(key + "_time", currentTime);
  return true;
}

var diffInSecs = Math.ceil((currentTime - applicationScope.get(key + "_time")) / 1000);
if (diffInSecs < cacheInterval) {
return false;
} else {
applicationScope.put(key + "_time", currentTime);
return true;
}
}

Use recycle methods

When running through a large set of Notes objects like with NotesDocumentCollection.getFirst(), getNext() watch the memory consumption of your HTTP task.
Since there are Java objects behind the JavaScript objects, then have the same requirement to call recycle() to remove them from memory.

Read Domino Designer help regarding recycle() methods of Notes Java objects.

Use firebug page speed

In Firefox you can add an extension to your firebug: page speed

.
This tool check where your website is slow and why. Very valuable.

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