This page demonstrates the default behaviour of Responsify WP out of the box.
The image sizes is set to the following:

Image size Width
Thumbnail 320 px
Custom: iPhone landscape 480 px
Medium 600 px
Custom: tablet portrait 880 px
Large 1280 px

Page weight
Loading times

Blog post 1

IMG_0566

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Blog post 2

IMG_2383

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Blog post 3

IMG_2370

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Blog post 4

IMG_6283

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Blog post 5

_MG_2029

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Page weight

Window width Page weight w/ RWP Page weight w/o RWP
320 px ~307 kb ~2.7 mb
568 px ~460 kb ~2.7 mb
667 px ~622 kb ~2.7 mb
1024 px ~1.5 mb ~2.7 mb

According to Google Chrome Developer tools.

Loading times, 3G connection (750 kbps)

Window width Load time w/ RWP Load time w/o RWP
320 px ~3.83 s ~30.51 s
568 px ~5.40 s ~30.51 s
667 px ~7.17 s ~30.51 s
1024 px ~16.86 s ~30.51 s

Based on simulation in Google Chrome Developer tools.