Just to expand on that a little, many programs will allocate enough space for the entire download when it first starts. You can disable that in some programs if you are really limited on space, but it does guarantee that there is enough space to complete the download without problems.
As far a speed, as you use torrents more, you will learn that speed varies substantially, and it is usually based on who is available to share. You are not downloading files from a server, you are downloading them from other people that are trying to download them. As you accumulate parts of the files, others newer to the torrent will be downloading from you and other users like you. there are seeds (people that have the entire file and choose to continue to share, so that the entire file remains available), but there are not enough to keep the torrent going. If there were only a few seeds, and no one downloading was sharing, your speeds would be so pathetic, that 2 gig file could take weeks, if there are even any seeds willing to seed that long (if not, you get part of the file and thats it, no more downloading). It can only work if everyone shares, and the faster everyone shares, the faster everyone gets it.
Use a speed tester like this one
To determine your internet connection speed. Try it at different times as connection speed can vary on shared connections like cable. Set you upload limit to about 70-80 percent of your max upload connection speed (leaving it at 100% or unlimited can actually cause download speed problems). You could set it lower, but remember, even if your download speed is slower than your upload speed, that just means that you are helping other get the file faster. there may be reasons that they cannot upload faster to you (not enough people, slow connections like dial up, they are sharing several files so their bandwidth is split up etc). there is probably someone on the other end that is very grateful for the fast speed they are getting from you so be nice and let them have it. I pretty much guarantee you the opposite will be true in the future if you use torrents much. You will get a torrent with blazing download speeds (for at least part of the torrent), maybe so fast that you couldn’t begin to upload as fast as you are download due to isp speed limitations. Someone is being nice and letting you take advantage of their high upload speeds, and you should do the same for others. The more and faster everyone shares, the better it works for all of us.
You also need to check your settings. Windows limits half open connections for security reasons, which can cripple your download speed. It is safe to increase the limit to help downloading. Try googleing azureus tips or azureus settings to find sites like this for tips on your settings.
Last thing, don’t get discouraged if some of your downloads are slow, I have had plenty that were very slow, as well as plenty that were quite fast. It just varies based on how many people are sharing that particular torrent, and their speed capabilities. There are plenty of people sharing that do not have broadband, as well as some that have unbelievably fast connections. I have gotten a few hundred KBps from a single user before and my connection is only capable of 60KBps upload speed. That brings me to my last point. even after the download has finished, keep uploading to help others (I do it all the time). It not only helps keep torrents fast for everyone, but it allows people to get the entire file. If no one that already has the entire file wants to share, then there is nowhere for you to get all of the file from (and some torrents do die because there are plenty of people sharing but no one want to seed). If a torrent is sitting dead like that (you have half the file and cannot get any more etc), you may want to consider an alternative torrent that has the same content. If you want to use the file right away, just stop the torrent, copy the files to another location, start the torrent back up and use the copy.