I am new to Joomla. i have to select one theme from these two themes.

Rocket theme - http://demo.rockettheme.com/?template=acacia
Nemo White Premium Joomla Template - http://themeforest.net/item/nemo-white-premium-joomla-template/5507845?WT.ac=category_item&WT.seg_1=category_item&WT.z_author=ThemeRox
but exactly i don't know which parameters i have to consider for selecting theme like Framework.

I am not sure how to check which theme takes lesser load times amongst other things

Please help.

What is you audiance, do you wanted to be responsive?To adapt to mobile desktop?
Are you going to use pictures?
After you answer this question i may be able to help you

yes i want to use this for responsive. for adapt mobile desktop also.my site is http://guru99.com/ you can check here it is site for educational purpose. So theme should be light so load time for that is also less.

After reviewing their option and taking into account their frameworks and brand I would go with acacia by rocket theme.
But you should ask each company if they real rezise the image or they are using fluid images.
To learn more about this google responsive designe art director problem
Hope this helps