You can look at the objects on the Objects tab of the sheet properties window the check the ram usage and calculation time of the objects. See the CalcTime and Memory columns. Maybe this blog post helps some too: Recipe for a Memory Statistics analysis. There is no specific tool for monitoring documents in the browsers afaik. Maybe there are developer plugins for your browser that can help.
There are three tools you can use.
1) save the .mem file locally from within an application which is not performing, use another tool to analyse the memory performance (object by object) this will help identify excessive consumption of memory for specific objects. (Document Analyzer is available for download I believe)
2) use System Monitor/Governance Dashboard (both free apps) to monitor CPU and Memory consumption over time. the Governance Dashboard has a great feature for monitoring data growth overtime.
3) use JMeter (we use this in our scalability labs) to monitor QlikView performance (the tool is open source) you run scripts (which are configurable) to execute simulations against your application and gather the results of the testing. You install the application gathering the application on a separate machine to the machine executing the QV application. It's free to use and download ( search for the scalability labs on community to find the download zip files) There are also a number of prebuilt applications with scripts created, useful for scaling your hardware.
Hope this helps you ?