A lot of people love R&S. It is a very reputable manufacturer, and the MXO series are a thing of beauty. The problem is, they are not competitive at the <1500€ price range, and they don't want to.
The RTB2000 would be the closest competitor, and it is a really nice scope. There are a few reviews around youtube, but it is certainly not lag-free. Really, nobody has managed to optimize interface speed and responsiveness as much as Keysight with the Megazoom IV.
The Siglent is more analytical. It's oriented towards a more thoughtful use, with longer captures and later analysis. It doesn't seem to be what you are looking for. I'm very happy with mine, but you will notice hiccups on the captured waveform when diving into menus, the acquisition will visibly stop when moving traces, math and FFT will slow things out, etc. Quite normal, just like every other DSO except, for the most part, the KS Megazoom IV.
Edit: Have a look at youtube reviews. Skip to the reviewers' twiddling of knobs and selection of different modes of operation, and you will get an idea of each oscilloscope's responsiveness. Other than trying a bunch of them out, it's the next best thing. Instant response is a thing of the past, the only manufacturer that kept trying to emulate old CROs was KS.