Everything working with disk storage will run significantly faster - you'll notice immediately. The smaller the files are the bigger the difference will be. With large files SSDs are these days at about 3x HDD speed. Once loaded and running from memory the difference will be close to none.
Windows - depending also on cpu, but in general starts to complete 'ready' state within couple of seconds
Games - loading times reduce a lot, games that use continuous loading might get less framerate drop spikes caused by disk IO waits (or reduce ammount of badly or not textured surfaces as the system now delivers the data much faster). Once everything is in the RAM, zero difference.
Software development - Visual Studio with Resharper and larger projects - several times faster, much more responsive, perf gap increases with the project size
Lossless video - what do you mean by that? Uncompressed video? Or H264/H265? 4k? If you are getting stuggering with blu-ray quality video it's either heavily fragmented or malfunctioning disk, sloware (hello Symantec Antivirus for one) or your computer lacks hardware acceleration for repective video codec.