Actually, it does have an effect on the site speed - a positive one, depending on the website traffic.
By default, Osclass is resizing all images using the integrated PHP libraries (GD or ImageMagick), which causes CPU and memory issues (mostly on shared hosting) if you have a website with lots of users that constantly post listings.
For every image added, the server needs to do some work to resize it.
The purpose of that plugin is to move the resizing work from the server, to the user browser.
So basically, the image is resized in the browser when the user uploads it, then it gets transferred to your server = 0 CPU & Memory usage, just minimal bandwidth hit.
Having said this, the plugin is actually a must-have for any high traffic website.
Edit: Actually there is some tiny memory usage as the Osclass core will still generate 3 images (Thumbnail, Preview, Normal), but not from the original image (which might be huge - 100MPX camera). It will generate them from the smaller resized one.