Downloads always start slowly and can appear to pause for no apparent reason. This is because the application needs to determine which encrypted data blocks belong to which requested files. The bigger your request, or the more total data in your account that needs to be waded through, the longer this will take. The time that this will take is non-trivial, and because of our No Knowledge framework this work has to be done on your computer, not on our servers. Once enough information has been gathered to start downloading an identified block, it does so. However identification of needed blocks continues, because downloading is usually faster than block identification. It is for that reason that it is common to observe a long period of "thinking" followed by a short burst of downloading followed by another long period of thinking. Blocks are downloaded to free space on your hard disk temporarily. Once all the blocks for a particular file have been downloaded, they are decrypted. That too is processor intensive, has to be done on your end and not on our servers, and leaves fewer processor cycles for block identification. Once a file has been decrypted, it is written to the location you specified, and the encrypted blocks that correspond to it are deleted from your hard disk to make room for more.
This has performance implications. You will want to perform this on whichever of your computers has the fastest processor, and if you want to maximize speed then you will want to minimize whatever other tasks might be using the processor. You will want to perform this on a computer with abundant RAM and free space on the main system hard disk. You will want to do the obvious thing of turning off any sleep or other power saving features on the computer so that it continues to work 24/7 even when there is no user input to keep it awake. And most importantly, keep in mind that this simply isn't a speedy task.
To be sure, we could have designed a system optimized for speed, and unload the resource intensive work to our servers. Instead we optimize for privacy and security even though it means that we won't be the fastest service in town.