As I'm sure you've come to realize, software development is a constant tradeoff.  Add more functionality, and performance takes a hit.  In the new generation of web applications, end users have come to expect a rich experience, and that doesn't come for free.  In many cases the performance hit is negligable, but there's always going to be a 'best case', where the feature-less will be faster than the feature-laden. 

Here are a few things to think about.

Using AJAX techniques, you can break through performance barriers that were impossible to break through in the past.  For example, you can use Virtual LoadOnDemand for the WebGrid to bind to millions of records, and have the performance of a grid with only 10 rows in it.  Rich editing is made possibe using masked editors, and can be done without annoying postbacks to "enter edit mode".  Paging can be done asynchronously, and the paging mechanism is built into the grid.  There's an entire client-side API that you can use to program against any of the Infragistics controls without having to postback.  Leveraging these aspects will enable you to create an application where the user can be more productive, and have a good experience. 

Here are a couple of additional points to think about when dealing when any web application. 

All script and css files are cached by both your webserver and the client browser.  The client will only have to wait while the script is downloaded the first time they visit the site (unless they clear their cache). 

Turning on GZIP compression on your WebServer can shrink the js files down quite a bit.

Also, you may have read in the beta forums that we're working on a new framework for our controls called "Aikido"  One of the goals of this framework is to minimize the difference between "feature" and "performance".  We want to deliver the features with the performance, and that's what we're focusing on. 

I hope this helps,

-Tony