I just got done verifying an process that I've heard mixed issues that result if your workflow process is not correct when using Revit MEP. And it's great news in my opinion.
When you place plumbing fixtures in your Revit Model, it is placed in the Default System.
You can connect multiple plumbing fixtures to each other with pipe, and get all the wonderful flow information - all without adding those fixtures to a specific system that you create. If you select the pipe and look at it's Instant Properties, you can not only see the flow going through those pipes, but you can see that it has a "System TYPE" of Domestic Cold Water and a "System NAME" of Default Domestic Cold Water. Make sure not to confuse the two.
The default View Filter displays System Types of Domestic Cold Water in a blue color. So everything looks good, but there is a limitation of how many plumbing fixtures you can connect to each other that are put on the Default System Name. And that limitation is 50 fixtures.
Below is a thread from Jason Martin at Autodesk explaining the history of this Warning further:
One of the things that is most expensive in the “calculation process” is maintaining the “pressure losses” in sections of a duct or pipe system. To do yhis we divide any connected system into sections at any junction or size change in the system (there are additional reasons for a section to be created, like a resistance change, and I’m sure a few others). For each of the sections we track things like pressure loss, static pressure etc. Maintaining these types of calculations for relatively small sets of elements is relatively inexpensive compared to the time it takes to do things like actually move the things. In systems with thousands of elements, however, these calculations became exponentially more expensive as the element counts increase. We decided at that point that when there are more than a set number of “system assignable-1” elements assigned to a system that we would turn off these types of calculations. After quite a few rounds of testing (as well as some arguing) we decided that a “tolerable” number of elements to turn off these “complex” calculations at was 50. So, now, as soon as the number of elements in a system goes above 50 you’ll get an error message that says something like:
“The default system "Default Supply Air" is now over 50 elements. To improve performance, Revit is no longer calculating the critical path pressure drop and the more complex duct sizing has been disabled. If you want to use these features, you must define logical systems in the model instead of using the default system.”
If you receive this error message it doesn’t have anything to do with things not updating in elevations / sections, and it doesn’t make the application automatically disconnect things. In versions prior to 2009 WU3 (or whichever one it was) it was very true that creating systems improved performance “massively”. Since the introduction of the “50 element” rule, it isn’t as significant as it once was.
For other “performance” things, it really, really, really depends. In 2010 a significant portion of effort was spent on improving the performance of what we referred to as “model manipulation”. This includes things like dragging, connecting, moving, sizing (either with sizing tools or just changing the size), and deleting items like pipe, duct and fittings. As a few others have mentioned here, “model manipulation” in 2010 is significantly faster than it was in previous releases.
If you are moving a connected duct / pipe / fitting - our internal testing has shown that it is actually “slightly” faster to not have systems defined than it is to have systems defined. If you are making new connections between things - our internal testing has shown that it is actually “slightly” faster to have systems defined than it is to not have them defined. If you are changing the flow of elements in the system (like air terminals) - it is “slightly” faster to have systems defined than it is to not have them defined. If you are creating systems - it is significantly faster to create them before things are connected with ducts and pipes.
System assignable elements are things like air terminals, mechanical equipment etc. Things that when you select them you see a create system button (or edit system button) but don’t include things like ducts, fittings, pipes, etc.
Which brings me to the name of the article, Don't Go Half Way. I hear a lot of firms that just want to model certain parts of the building in Revit, and do 2D for the other in AutoCAD. Or they want to just create a 3D model, and not use the flow calculation tools that Revit has. But there are a lot of things going on in the program that if you're not aware of, you run into these limitations such at connecting 50 elements to a default system and you end up creating more headaches than you thought you were going to avoid by taking shortcuts. So go the extra step and create the systems and use them to your advantage. You can save so much more time accepting the full Revit process the way it is supposed to be used, than trying to come up with workarounds and unproven processes of how Revit is not supposed to be used.