I don't really grasp what you mean by it being slow. I have a TDS3054 and a TDS784C and the only time I've ever noticed any kind of slowness in either one is using deep memory on the TDS784. The TDS3000 feels very snappy to me, what do I need to do to see this "painfully slow" lag you refer to? I'm genuinely curious and don't know what you're talking about.
It's not about 'lag' or general controls. There isn't any input lag when operating the scope. But unfortunately the user interface isn't everything.
For example, try mask testing on the TDS3000. Or FFT. The TDS3000 is also slow when it comes to waveform rates, as in normal mode it's trigger rate is some 450 wfms/sec. This raises to 3k wfms/s or so in Fast trigger mode but then the sample memory (with 10kpts not exactly large) is limited to a measly 500pts. It's not a big problem if you can make it with the available trigger suite (which is quite good if the advanced trigger option is installed) but that doesn't change the fact that the scope *is* slow, and when used in an 'analog scope' manner (like searching for glitches through trace persistence) then it will perform poorly.
The TDS784C isn't much different, it's older and even slower. In normal mode the waveform update rate is some 150 wfms/s with a 500pts memory and infinite persistence. InstaVu raises the update rate to some 400k wfms/s, however this is purely a viewing mode (i.e. no math, no FFT). This isn't much of a problem as rare events can only reliably be found through triggers (even at 400k wfms/s your scope is blind >90% of the time) but still, it's slow. This is also visible in when running FFTs, mask tests or some more complex maths, all which slows it down, even though FFTs are quite small (64k? can't remember) and measurements are limited to just four.
The other thing is that if any Tek scope is busy doing stuff then it locks up the user interface.
While I do like the TekProbe interface I'm not opposed to other scope brands, but aside from the very shallow memory depth by modern standards and the exorbitant pricing on the still-available C version I really haven't found any other complaints about the TDS3000 series. It's the perfect form factor and the controls are laid out very logically, I can set it up with my eyes closed.
Well, glad you like it, and if that's what suits you then that's perfectly fine of course, I'm not saying you should move to something else.
But this is about the question why Tek has lost the plot so much that today they represent the bottom-of-the-barrel when it comes to A brands. And for that it's important to look at the big picture, i.e. not the self-congratulatory stuff on Tek's marketing brochures you seem to be so fixated at, but at how Tek's offerings compared to the competition and what the market wanted. And it's obvious that Tektronix wasn't very successful there.
And this is for several reasons. For one, Tektronix never really understood the implications of going digital with scopes. Back then when the first early DSOs came along, Tektronix has long been very successful with its analog scopes, and more or less dominated the market. It's engineering base was very loyal, and more or less agreed of the "Tek way" of what makes a good oscilloscope. Digital scopes were at first considered a short-lived faff, then a niche product. Later, when Tek decided that it, too, needs to offer digital scopes, it basically tried to copy the behavior of analog scopes, which was seen as the most important thing. And this is reflected throughout Tek's DSO portfolio over the years.
The problem however is that Tektronix never realized the potential which comes with digitization, all it wanted was making analog scope like digital scopes. In reality, analog-like behavior was certainly a driver for a less expensive standard bench scope, above that however customer expected some new advanced analysis and measurement capabilities. HP and LeCroy realized that, while Tek kept its focus on visual gimmicks like InstaVu and at the cost of functionality, missing the mark by a long shot.
The other mistake is the belief that the brand name would have enough pull to guarantee sufficient sales. Well, it didn't, at least not long enough. Customers were quite aware that over the same time the competition came out with a series of new products Tektronix pretty much just offered a simple re-has of its existing product lines. And this isn't just true for the TDS3000, the whole TDS line was dragged along, with little improvements, way beyond it's best-by date.
Since DSOs became a thing, most of the innovation happened outside Tek. That support quality also declined dramatically from the old analog days just helped to accelerate the downward spiral.
A couple of years ago when the MSO6 came out I had great hopes that the new Tek, being freed of Danaher, is turning things around, but the scope was as lackluster as it's predecessors. Rinse and repeat for all the new models.
It's a shame really, not only because Tek once made the best scopes (when they were analog) and it's quite sad to see the constant decline, it's also bad for us customers because it means there is one less viable competitor out there which could help to keep pressure on price excesses. Unfortunately Tek seems to be very ambitious when it comes to pricing, and we're now at a point where mentioning "Tektronix" when talking to a sales droid of any other T&M manufacturer only gets you a sad smile.