If you use an Android device, TubeMate is an excellent app for this.
-launch TubeMate
-find what you want to DL on YouTube. Get the video showing in the run window of YT
-click the TM button that's overlaid on the bottom right corner of YT
-a popup appears in TM to select download resolution/file size. Pick what you want, then click the red DL button on the right side to start the DL.
-it'll dl at whatever speed it can achieve. For instance, a 180MB 1280x720 file took under 3 minutes.
Alternately, if you aren't seeing the TM button on YT, in YT select Share - Copy Link, paste the link into TM's address bar, then tap the DL button to have it open the video resolution screen.
Files get saved to the Internal Memory\Video folder on your device. You can move them to another folder (I use VapingVideos on an SD card).