do thumbnail image styles lower the page load
Uploaded images are scaled depending on the preset you choose for display. Thus, a typical high resolution image which might have 8 MB is reduced to maybe 150 kB when using a thumbnail preset. This of course lowers the bandwidth, the original image is not loaded. 

So, there is no need to create a second image field. If you haven't seen it yet, you might want to take a look at the Colorbox module. It lets you choose to display an image as a thumbnail and only upon clicking on that preview the full image is loaded (or just another size you defined).

Just a note: Colorbox is not the only module that is providing that feature, there are a lot of similar modules, the original was called Lightbox. On drupal.org, there's a page comparing all available lightbox clones.