For what it's worth - I use my MDO4000C nearly everyday, and don't find it to be slow, sluggish, clunky, etc. You'll see the MDO in dozens of my YouTube videos, and I doubt that any of them show any sluggish behavior.
Now, can you make it get sluggish? - sure you can. Because the scope gives you the ability to directly control the waveform record length *AND* the fact that waveform processing (such as waveform measurements, Search & Mark, etc.) operate on the full waveform record and not a decimated set of display samples - you can make the scope "sluggish" by increasing the record length and turning an operation that pounds through the entire record (like Search & Mark). Similarly, asking for a very narrow RBW on the spectrum analyzer will result in a huge FFT vector length which takes time for the embedded processor to crunch. I do wish that the UI buttons were handled in a separate processing thread, this would help the UI responsiveness in this corner case.
But for everyday operation and use cases, I find the scope very responsive and lively.