On both computers, with VSync disabled, I reliably get an FPS/UPS of 60/60 (an ideal value). Both computers can handle larger factories without problems, without suffering any drops in FPS or UPS.
However, when I enable VSync, my older computer drops to an FPS of about 40. UPS is unaffected and remains at 60. My newer computer does not suffer from this drop in FPS.
In Factorio Friday Facts #70, Kovarex wrote that a FPS/UPS of 40/60 is the result of rendered frames not being prepared in time for the vertical synchronization point. He states that this happens when many things are on the screen that need to be rendered and that this could occur especially on far zoom levels.
However, on my older computer with VSync enabled, I always have an FPS/UPS of 40/60, even at the very start of the game where there is very little to render and even if I zoom in fully and set the graphics to minimum ("sprite resolution" to "extra low"). Therefore, I do not believe that my older computer is unable to deliver 60 FPS, but rather suspect that there is something wrong with the game loop. The fact that my older computer is able to reliably deliver 60 FPS with VSync disabled seems to corroborate this.
I am using the latest Nvidia graphics card drivers on Windows 7. I have been experiencing the issue at least since version 0.13 and the issue still persists in version 0.15. I am not using any mods. This issue is also not related to a specific savegame. Even if I start a new game, I have the same issue right from the start.
Strangely, this issue disappears completely when I start the game with the launch option "--force-opengl". In that case, I always get a FPS/UPS of 60/60, even with VSync active.
What is also strange is that when I set the the launch option "--force-opengl", the Factorio setting "Wait for VSync" seems to have no effect. I notice no visible tearing either way. Therefore, I assume VSync must always be active when using OpenGL, even if I disable VSync in Factorio. My Nvidia Control Panel is using default settings, i.e. it is not overriding VSync.
I have attached 4 log files, the first are with VSync enabled and disabled in Direct3D, the next two log are the ones in OpenGL.