If you're talking about videos and hard drives vs. SSDs in general, videos are typically large files, so as long as the drive isn't fragmented, the data can be read sequentially. Hard drives are far worse than SSDs at random access, but they're not too bad at sequential access (reading sectors of data that are right next to each other). Applications and operating system boot time improve more with SSDs because they load data from all over the disk.
If you're talking about your particular installation, it's possible that the problem isn't the speed at which data is read from the disk, but the speed at which your computer decodes it. If your CPU or graphics card isn't powerful enough to decode, say, a 4K video file, it doesn't matter how fast your SSD is.