What's your Big O efficiency in terms of request and delivery of chunks?

I'm not sure what you mean by that, but I think the answer is 'linear in the size of the data'. A 1GB file will take ~10x as long to download as a 100mb file. The bottleneck right now is generally uploader bandwidth, however after the next release it's much more likely to be your own bandwidth. In terms of memory it's constant size, something like 50mb of RAM to do the download process (regardless of file size). Disk space is just the size of the file you are downloading. CPU usage is a more complicated question, but it boils down to "so fast you don't need to worry about it unless you've got a 10 gbps connection piped straight to a single processor and a high end SSD to match".