My problem isn't the same as slowing down to 15fps, but the game still stutters, quite a bit with the HWAccell On.
With HwAccel off is smooth.
I've been trying any combination of options since i've posted here my comment.
When you set the videodriver = "win32" line in the config file, you force that specific driver and skip auto-detection. The "Use hardware acceleration" option in-game only affects the auto-detection when no video driver is set explicitly. So you're just forcing the setting to "off".
The hardware acceleration is (currently only) based on OpenGL, and it has some components that are shared between the OS drivers (Win32, SDL, Cocoa) and some that are shared within the OS drivers (parts common to Win32 software and Win32 OpenGL, for example).
OpenTTD's Win32 and Win32-OpenGL video drivers have the parts in common that create the OS window, handle input, and run the basic input-simulate-output loop, while the part that actually puts pixels on the screen are separate.