Note that when using Ajax, the client connects to a web server, (QVWS or IIS) and then to the Server, and back again. So network time increases considerably. And the amount of information is larger, since it has to encapsulate the information from the server to the web server, and from the web server to the client. If browser have some filters or extensions will affect, too.
That's why using the IE Plugin via QVP is quite faster.
The expressions and how the data model is created also influcences: using If() conditionals, using string literals instead of numeric values in set analysis, or calculated dimensions...
But charts draw fine in Desktop, then it's likely a network/browser issue. Check whether opening the document using Ajax in the Server is so slow.
Hope that helps.