When you have Vsync enabled: Vsync is used to synchronize the output of your graphics card with the display of your monitor. When your graphics card has finished rendering the next frame it waits for the monitor to finish displaying the current one before switching to the new one. This means that the maximum frame rate you can obtain will be equal to the refresh rate of your monitor (which is usually 60 Hz, 75 Hz, 85 Hz, or 100 Hz).If you disable Vsync, then your graphics card will continuously render without waiting for the last frame to be displayed in its entirety. With fast graphics cards this means that your monitor may switch to a new frame halfway down the screen. This effect is known as tearing as there appears to be a visible line separating two different halves. Due to this, you should generally leave Vsync enabled except when benchmarking.Read here

When you have Vsync enabled: Vsync is used to synchronize the output of your graphics card with the display of your monitor. When your graphics card has finished rendering the next frame it waits for the monitor to finish displaying the current one before switching to the new one. This means that the maximum frame rate you can obtain will be equal to the refresh rate of your monitor (which is usually 60 Hz, 75 Hz, 85 Hz, or 100 Hz).If you disable Vsync, then your graphics card will continuously render without waiting for the last frame to be displayed in its entirety. With fast graphics cards this means that your monitor may switch to a new frame halfway down the screen. This effect is known as tearing as there appears to be a visible line separating two different halves. Due to this, you should generally leave Vsync enabled except when benchmarking.Read here

I think everyone in this thread already understands what vsync does....its the side effects and best place to enable it that is being discussed here.

I indeed know what Vsync does. The strange thing though is that below 60 fps there are stutters for me (and some others and apparently a lot of people who fly @ fps 30 don't experience stutters. Is it that my eyes see things that others don't see or is it a hardware / software issue?

Top respect to NickN for his knowledge but the firm assertions that are often made, don't always prove to be the case for every user on every PC config. There's often a hard view on the science, but sometimes application can change the results of that science! On my system (which is an older C2Q E9650 3.0 running XP), disabling Vsync (whether in the nVidia Control panel or in FSX as per Bojote) does reduce stuttering, and raise FPS by 3-5. Similarly doing what Rob Young suggested and raising FSX priority has given me further smoothness, and another 3 or so FPS. But......... the priority setting tweak works on all addons for me .........except the PMDG NGX, where it leads to a fast CTD. The single best thing that improved performance for me (and allowed me to actually use my NGX at all), was installing antilag into the FSX root folder, and locking frames in it's config file to 30. (FSX internal limiter shouldbe kept on unlimited). Do a Google search for antilag fsx. As always the key is -------> YMMV. Cheers,

Similarly doing what Rob Young suggested and raising FSX priority has given me further smoothness, and another 3 or so FPS. But......... the priority setting tweak works on all addons for me .........except the PMDG NGX, where it leads to a fast CTD.

That makes sense, since complex aircraft with a lot of background gauge and other calculations probably need lots of RAM and processor devoted to them as well as the core sim. But I would guess that is a rarity. I've never seen any crashes as long as I do not raise priority to max. The smoothing out of those regular stutters is palpable, not imagined. As for affinity (answering several posts above), I've tried every combination and frankly see not a blind bit of difference, but that of course might just be me. I'm very keen on being placebo-aware. Rob - RealAir

Non Vsync tearing and microstutters are different things. I got rid of the regular small stutter of the kind the OP described by doing the following (folks like Nick think this is just covering up the problem, but it fixed it for me): With FSX running press ctr alt del and bring up the task manager. Under the processes tab find FSX.exe. Right click and set priority to one notch higher than normal, but not the highest, which could crash the sim. You will find toggling the FSX map will be a little slower but this is the only trade off and it's acceptable. I do this every time I run FSX, and I have never had a single, pulsing, regular stutter of the kind you describe since doing so. Before doing this I've had this stutter on every system I have run. It might not work for you, so apologies if this is the case. It is worth a try though. Rob - RealAir

I concur w/ the value of this for microstutter management. In addition, it is critical (on many if not all systems) to understand the impact of the visible mouse pointer, which will rob a good 25+% from frame rate. What I have done w/ the NGX and RealAir Turbine Duke, both of which suffer visibly from this issue on my machine, is leave the ATC window open and 'park' the pointer on it. You can do it w/ the menu bar as well. Parking the pointer on either of these windows prevents the degradation of performance and when the pointer auto-disappears after a few seconds, I can then close the ATC window. This is a very sorry bug and I wish to heck someone who understands FSX could find the cure for this. I know Rob has tried to explain it to me, but it's hard to follow. Its seems an unintended consequence of the specific programming involved. Rob, you can use the nifty utility, Prio (prio_x64_199_2091.exe) to allow you to save both Affinity & Process Priority to any executable, and you won't have to go in and do that step w/ each launch of FSX. Noel