Thanks for chiming in tlw. I was running daws in the 90's too and recall all of the nightmares involved! We certainly have it pretty good with modern computers. Did you try the experiment that I suggested? I am not suggesting that this is the way to add reverb to your tracks - I use exactly the same methods you describe. I was just surprised to find that sending multiple tracks to a single reverb used roughly the same amount of cpu as adding individual reverbs to the same number of tracks. In a way, from a digital perspective there is some logic to it. Each track still has to be processed. 
 
Just a quirky observation that has probably taken up too much of everyone's time:)

It's hard to tell,  tenfoot, if you're fully satisfied by the answers or not. It sounds like your question is subtly related to two different performance-related concerns, one being the overhead of routing and mixing, and the other being the effect processing.
 
In the case of a single effect on a bus, the 6 audio streams are mixed into one and processed by a single pass through one instance of the effect. In analogue terms, the signals are mixed onto an aux bus and passed into a single input on an effects box and the output is mixed back in on an aux input or channel. Then the output of that bus is mixed with the rest of the project and sent out the main bus.
 
In the case of effects on 6 tracks, separate audio streams are passed separately into 6 copies of the effect, each requiring a pass of an audio stream through the algorithm. The CPU used for the effect is thus six times in the previous case. Then the six effect outputs are mixed into the rest of the project. In analog terms, you would have to buy 5 more boxes to do that processing using channel inserts. The analogue vs DAW reasoning is somewhat similar: you want different parameter values in each of the 6 effects.
 
The routing and mixing overhead is virtually the same in the two cases. You could argue that there is an extra bus required in case 1, like the aux bus on a mixer, but the actual CPU used by the existence of that bus is negligible. At most it requires an extra step in summing the audio stream from that bus into the main mix, whereas in case 2, you could sum all audio streams at once into the main bus mix. But in technical terms, mixing audio streams is negligible compared with processing audio streams through an effect.
 
To complete your experiment, you would need to run your project with no effects, then add the one effect to see if that effect is using enough CPU to be measurable. Then your 6-effect case would be easier to interpret. In other words, first find an effect that uses significant CPU, then try six of them. You could also separate your measurement of using the extra bus or not, by running the project with and without the extra bus, but with no effects, to see if that difference is measurable.
 

Yea, just keep adding Perfect space to a project and keep an eye on the usage.
 
@

Thanks b video - that's a brilliant explanation and indeed what I would have expected to be the case. Just to be clear I have understood, given that the routing and mixing overhead is the same in both cases, and that using six instances of the effect algorithm will logically use 6 times more processor, I should be seeing a difference in cpu usage between the two scenarios. In my previous test, I fully expected this to be the case, but I saw no such difference. 
 
I never expected the additional bus to cause any extra load. I did expect the six separate effects to use more cpu than sending the same 6 tracks to a single fx bus. The trouble is - they just didn't.
 
To be clear: Inserting a reverb on a single track used roughly the same cpu as sending a single track to the same reverb on an fx  bus - no surprises there. Inserting 2 instances of that same reverb on separate tracks used roughly the same amount of cpu as sending the same  2 separate tracks to a single instance of the same reverb via an fx bus. That I did not expect! This correlation continued as additional tracks were added.  In light of your explanation it just should not happen. I should see an ever increasing difference in load between the two methods.
 
I will try with a different plugin when I get a chance. Perhaps there is something peculiar to whatever effect I was using. 
 
Thanks again for your time - much appreciated:)
 
 
 

Yes, the load of separate plugins is additive. To see it on a high-powered system with a lighter weight FX, try this:
 
- Create on audio track and insert your chosen FX on it.
- Set the Fader to -40dB
- Make 99 clones of the track (yes, I'm serious; the output of 100 tracks at -40dB will be equivalent to 1 at 0dB)
- Watch what happens to your performance meter in SONAR when it finishes cloning.

Thanks for your input and patience everyone, and for the detailed explanation and detecting my unease  bvideo - I think that I am a step closer to getting to the bottom of this.
 
I have 3 DAW setups that I use; a main studio rig, a live rig, and a backup live rig. The system I have been testing on is the backup live rig. I chose this particular system as it has the lowest specs and is easiest to see any change in cpu usage. If I create a new project, import a single track of audio, insert a reverb into the track fx bin, then clone the track 30 times, I see the cpu load go up significantly, exactly as you would expect. But here's the thing that lead to my confusion; If I create a new project, import the same audio, create an fx send on that track and insert the same reverb over the fx bus, and then clone it 30 times, I see exactly the same increase in CPU usage as before, despite there being 30 tracks feeding 1 fx bus.
 
However I then repeated  the same test on my studio system and live rig, but this time the results were exactly as you would expect and everyone has described - the cpu load with the individual fx was much higher than with the individual send. A breakthrough! It is clearly something on that specific system causing the errant behaviour. My search has begun!
 
Thanks again for all of your input. Due to my ridiculously small research sample of 1 and resultant dodgy conclusion, I would have been left with a niggling feeling that something was bizarrely skewed in the world of digital fx processing if it weren't for your help. Who knows how that may have ended - awkward at parties at the very least:)
 
This forum is brilliant!
 
 

Just a thought, but for some reason my copy of Platinum when I first installed it turned out to be only using two of the eight available cpu cores (out of 4 physical and 4 virtual ones doing the hyper-threading). It was also showing very odd cpu usage behaviour as plugins were added or removed. It turned out the solution was to go into Sonar's preferences and tell ot to create a new aud.ini file. My guess is the problem was due to settings having been successively imported from x1 to Platinum via X2 and X3 and their sub-versions with something finally not transferring over as intended.

It might be worth a try in your case. As might checking Window's core parking is switched off.

Thanks for the tip Tlw. 
Core parking is disabled. I will definitely try the aud.ini file.