The speed of our service relies on 2 factors: your internet speed and Amazon's AWS server load. While the first may seem obvious, we keep our service on AWS infrastructure so if they experience issues or higher traffic then it affects CloudApp download and upload speeds.
Another thing worth mentioning which affects upload times of screen-recordings or multiple files: if you record a GIF, our app has to process it first (GIFs, are more like a 'set of screenshots' put together rather than a video) which slows down the upload speed. This doesn't happen with HD videos. You can do some ~20 sec. samples of both to see the difference.
A similar thing happens when you upload multiple files at once - CloudApp has to compress them into a ZIP file which takes time and adds to overall upload time.
If you have further questions about this, please contact our support team here.