I'm using IrfanView 4.66 x64 on Win10. I go into menu > File > Slideshow. I add files, largely go with default settings, and click Save slideshow as Video. There, my settings are width 1036, height 1928, image duration 0.04, blending duration 0.00, fps = 25, 8Mbps. With hardware acceleration, the output MP4 (according to MediaInfo) is about 16 Mbps.
