My iMac is eating my bandwidth

Thanks for the thoughts and time. I am not certain we are on the same page here. To review, when my iMac is on the net, I often (not always!) get very long pings, 500-1000, and the download speed often goes to near zero on any device on the net including the iMac. I have several other computers potentially on the net and none of does this. When I take the iMac off the net, I have a very steady 2.8 mbs with a ping consistently less than 30. I live in "the sticks" and am lucky to have any DSL at all. 2.8 and 30 is the best I have ever had! I guess it is because only the iMac is doing this that i think that there is something screwy with the iMac causing this fault. What am I missing? TIA

Sorry, I missed this answer at first. You are right, I have a lot of collected stuff I never use and more that I do not need. I am leery about erasing, just don't want to lose anything. I have ample EHD space for all the music, movies, photos, docs. etc. I fact, it is already backed up there and on iCloud, google Drive, and Dropbox. I seem to have saved all over the place. The thought of starting anew is daunting and appealing. What is the best way to save the apps I think I will need? Is there a good step by step site you suggest?

OK, I did the fresh install without migrating anything, just the "off the shelf" stuff, and I am back where I started! This really surprises me as I was hoping (believing) that getting rid of years of unnecessary things would fix it. Not so, it seems.

Could this be as "nsurlsessiond" issue?

Here is "photos" preference:

Here is activity monitor:

I am thinking that there is something else using nsurlsessiond, but I don't know what. I recently updated to Yosemite 10.10.3 when this began. I also have Dropbox, google drive, and iCloud drive accounts.

TIA!!

Thanks for that! I have disconnected from iCloud, signed out. Here is where activity monitor says what filesd are being used:

Is it possible that there is some kind of "closed loop" thing going on?

I think I am creeping up on it! After logging out of iCloud, I rebooted the iMac and now have this:

The little bursts are when I checked the mail as a test. Now, the question. If after all this I really want to use iCloud, what can I do to stop the giant sucking of my meager bandwidth?

Very, very helpful. Many thanks. Now that I understand better, I favor continuing to use iCloud while restricting activity to late nights and turning it off again in the morning until all the uploading and downloading is done. I have a large collection of photos it was trying to sync as well as other stuff. The mystery of my bandwidth is solved and I can cope now. Again, many thanks!!

You ask an important question about using iCloud. For those of us with modest bandwidth, it is an important "sleeper" issue. iCloud is touted as the solution to all syncing problems allowing complete cross platform content maintenance, but the bandwidth cost is not often mentioned. On most days, my DSL will give me 2.8 mbs of service. I have 2 devices, the aforementioned iMac and a traveller MBA. My wife has an iPhone, iPad mini, and MBA. That's a lot of info sharing if you go the full route and a lot of bandwidth consumption depending on what you are sharing. I have come to the conclusion that iCloud, wonderful in concept, is simply not practical for my situation. I have turned it off and will use Google drive and Dropbox for some of the storage duties. We will live without syncing photos automatically. Having "grown-up" with dial-up and then thrilled to unexpectedly have DSL, I have no problem living at 2.8 mbs. I have VOIP and stream TV, but iCloud is not for that bandwidth as best I can tell.