[joshvm]

Yes, all the sampling is performed on the Picoscope and then streamed over USB.

This is something that you need to grok when using scopes: you're capturing (often) a teeny-tiny snapshot of time. The waveform on your display might be 50ns wide. You certainly can't process this data in real-time using your eyes (that'd be 20 million frames a second). So you're not seeing a continuous signal, you're seeing short snippets of your signal separated by some dead time. The majority of this dead time is probably just writing to the display. You're not pushing pixels to the display at the sampling rate of the scope, otherwise these would be some of the best monitors in existence!

Usually scopes are triggered somehow and you'll be sent waveforms, typically centred on the trigger point, at some maximum frame rate. So you'll see a little bit before the trigger and a little bit after. Most scopes let you vary the trigger offset, e.g. if care more about stuff that happens before or after. Using an oscilloscope well is all about the triggering!

The scope will have (a) a max. sampling speed and (b) a fixed size sample buffer. You trade off both. You can sample really fast for a short period of time, or slower, but see a longer time period. If you want to see 'all' the signal, then you can set the timebase to something long - e.g. 100ms/div. That's useful for seeing slowly varying signals, but you'll lose resolution. A scope with a nice big sample buffer will let you zoom in on a signal once its been captured.

Once the buffer is full, the scope has to transfer that data to you, either directly to a screen or over a serial connection. Unless you're in single shot mode, this process will repeat every time the trigger condition is satisfied.