A new option has been added to ModelConverterX, with this option you can force the preview to use simple shaders. This option has been added because certain graphics cards have performance issues with the more complex shaders that are used by default. By setting the ForceSimpleShader to true the preview is forced to use the simple ones (you need to restart to see the effect).
What does simple shaders mean? It means that the preview will not show the more advanced features like night textures, bump textures or specular textures. Only the diffuse colour and texture will be shown. But that should be enough to view your object at a better performance.