gifv's are literally gif files converted to video.
Video is easier to load than gif files. And most gif files are stored as image-by-image completely seperately for each frame of motion. So it's like downloading hundreds of images then playing them in sequence (Some gif formats don't do this, and are lighter, but it's still large)
video formats of today (h264 / h265 video formats) store only the frame differences in video streams, each frame only changing what needs to be changed. So if you're downloading your favorite cartoon you might be able to fit a 1080p episode in just 65mb, because most scenes have static, non moving backgrounds that this format recognises as non-moving, and therefore doesn't store data for it after the first time it appears (it just leaves it up)
So there's a lot of efficiency with a video vs the gif 'slideshow' format. I personally, at home only have 500kb/s download speed. and most websites do NOT take full advantage of that speed when transmitting a gif (it's as if it's sent to you frame by frame too) But a video on youtube? YouTube have many automatic quality adjustments and all in place to make sure your video doesn't stutter, even if it has to kick the quality down a gear.
Gifs literally cant do that [you get what you see, so if your internet's slow, too bad no quality options] and people still think it's a valid format in 2016.
That said, internet should be good enough to handle this, and in cases of people with fiber internet or NBN in Australia they probably won't notice. But the rest of the world does.
Gifv formats on imgur, and the website gfycat in general are designed to combat this problem by converting uploaded gifs to video behind the scenes for you to use .gifv as a link option. In some cases you can turn a 50mb gif that goes for 10 seconds into a 5mb video just because it stores the video better (Who'da thought)
It can get controversial though. Many OPs are unaware of this.
EG: for karma, people will often trim down a specific part of a youtube video or something, and then upload the 'interesting part' to imgur as a gif and post it to reddit.
I have no fucking idea why you wouldn't keep the original video format during this, but people still like gif's that much for video files. And often this leads to a loss in quality due to the storage format of gif files. Let alone, an OP, intentionally lowering the quality so they can upload the gif faster or something. (this all happens, it's sad). At least on these sites there are options to upload video now somewhat combatting the shitty quality gif + slow loading problem at the same time.
It's an old format that needs to die. Video formats like h264+5 do it's job better AND with sound...