There are a lot of factors here, but let me lay down some basic rules.
1) 3d gauges perform more smoothly and are more responsive and FPS friendly than their 2d counterparts. This is a true statement, because movement is based on an animation versus pure coding. Almost all glass cockpit simulations perform worse than their 3d counterparts. Flight1 Mustang, PMDG MD-11 and 747-400X, FeelThere/Wilco Tiltrotor, etc. I could go on and on. What it has to do with is the amount of CPU cycles and checks necessary to complete the same task. A great example is the default C172 versus the C172 with the G1000. If for example the A2A 377 was built entirely in 2d, the plane would lose easily 5 FPS, and more than likely more.
2) Combined with the above, background tasks on your computer can cause major and intensive CPU cycles to get bogged down. For example, try doing a search for a hidden file on your computer while trying to record a DVD. A single core on a CPU can only handle one task at a time, and then from that prioritizes the tasks to make the work efficient. The truth still however is that when you are trying to cram code down the CPU pipeline, it is trying to cram millions of other lines of code for the game, the OS, background tasks (such as auto update, firewall, virus check, etc etc etc.) This is also why the advent of the GPU was so important, it takes the calls to draw 3d architecture off of the CPU and places it on the GPU. Another reason why 3d gauges perform better.....
Having a clean computer (free of any unrequired background tasks) can do a lot to improve performance. Things like defrags, and removing unnecessary drivers can also go a long way to improving not only the performance in FS, but in ALL games. Even something as simple as having your FS on the same hard drive as your FS install can bog performance as along with the CPU, your hard drive can only do one thing at a time. This is important when looking at multiple hard drives as well as multi core CPUs.
3) Polygon counts. While FSX can contain as we know it an unlimited polygon count, each polygon takes a cycle on your CPU and GPU. Higher polygon counts mean that the computer has to work harder to render the image. Computers that can run FSX are able to handle very high polygon counts (500,000 and up, not including the aircraft), however eventually with a high enough polygon count it simply starts to crawl after a while.
4) Texture resolution can make a difference depending on the bandwidth your GPU will allow. On similar systems, the GPU can make all the difference here.
5) Sim! This is a big one. In FS2004, calls for things like HUDs can reduce refresh rate by half for every instrument. This was lifted in FSX, however if models are not done through the SDK for FSX, issues can still occur. Not to mention that models made through the FSX SDK simply perform better anyways. That is why port overs perform worse than their true FSX counterparts. This may also explain some of the problems with the Iris F-15 HUD. It is all in the coding baby, but let me warn you that what may seem like a simple HUD is thousands of lines of code and often months of work.
To sum it up, there are a LOT of factors not only based on the computer, but the coding. FSX is a tough cookie to crack. While some may find performance acceptable or quite fine, others may find that there are huge FPS issues on even mundane aircraft. This creates major issues for designers as there is no set of true rules that can fix everything. While for example with the Iris plane I see very little performance drop and find a larger hit from the Aerosoft model, others are seeing almost exactly the opposite.
There is also a double edged sword here too. People want FPS friendly, but that often comes at the cost of systems or visual representation. That is just the reality of it all. The people that are fairing the best in this regards are the folks that are able to model their planes in 3d, including gauges. For some planes, like the F-16, this simply isn't an option.
This is also why we may never see a full glass cockpit plane with all the widgets you might for example see in an Accusim or Lotus product without a significant FPS drop. Until faster computers (significantly faster) or a better sim with better optimization of coding, calls all the while taking advantage of the available hardware, we are stuck between a rock and a hard place.
Very well put DSWO 
