When I started out with Double handed rods, & learned to Speycast some 30+ years ago, DT lines were all we had available so that's what we used in floating, sink tip & full sinking formats (most sink tips were absolute swines to speycast, & when intermediates came out they were greatfully received for fishing below the surface but not too deep).
I believe learning with a DT line made you a better caster, but it was a much longer learning curve. It's possible to teach a reasonably mobile & coordinated beginner to cast sufficiently well to cover a reasonable distance (sufficient to catch fish) in a morning with a Skagit outfit, a bit longer with a Scandi set-up & longer again with a mid belly Spey line; but a DT line takes significantly longer to become even semi proficient with as you simply don't feel the instant rod loading you experience with short but heavy for length style shooting head format lines.
Remembering back, my observations are that it's much easier to cast DT lines well with a longer rod, i.e. 15 feet or longer, & that rod should have the appropriate action with a stiffer tip & a relatively full flexing action which will bend right down into the handle (but I don't mean a slow or sloppy rod, it can still recover quickly, but needs to flex deeply under load). So something along the lines of a B&W Powerlite Speycaster, CND (I have an old CND Expert 14 footer which is a good rod for longer belly Spey lines or DT lines), some of the CTS blanks or similar actioned rods. Soft tip rods which only flex in the top 2/3rds in tended for Scandi heads aren't ideal with DT lines & really struggle when you are trying to dig out a deeply sunk full sinking DT line.
You need to be able to lift and hold a large energised loop in the air because a DT simply won't shoot line as far as a head or extended WF type Speyline with their' much thinner running lines; so to achieve distance you have to work with more line out & lifting this requires a rod with sufficient back-bone + more effort on the part of the caster than fishing with a short head line format does. It is vital to put the effort into the lift & back swing so the loop is held up in the air with only the tip of the line/ leader in contact with the water, then the forward cast can be executed with much less effort as the rod is already partially loaded before the forward cast is executed. If you let the loop lose tension & the lower leg of the loop fall onto the water then no amount of grunting & trying to over power the rod will recover the situation & turn it into a nice neat & straight cast.
Thinking back to then, I was in my late 20's when I started out with big rods & DT lines, I would fish from dawn to dusk with them for a full week & feel fine. That was then, I'm turning 60 on my next birthday & when I go away for a week's fishing now it's a more gently paced week & I'm really glad that I can rig up a 13 or 14 footer with 9/10 weight heads & cover the river because I simply don't think I could manage a full week with the 16 footers & 11/12 weight DT lines - at least not if I wanted to enjoy the week anyway.
But just now & again, it's fun to dig them out & have a couple of hours with the old kit; & then it's equally nice to put it away again & pick up the NRX & a shooting head! (Hopefully before I pulled a muscle waving one of the old brute sticks around).
But just remember, a rod designed for shooting heads won't make the best out of a DT line, & vice-versa; and some rods are easy to cast with (with a full floating line anyway) but just run out of steam at about 80 feet range - so if you are struggling with a DT line it may be worth trying the line on some other rods designed for long head/ DT lines as you may have reached the limits of a particular rods performance. If so then trying to push it harder won't help. The slower & softer rods can be easier to learn on, but once you have reached the edge of their performance limits then the only way to progress further is to up scale the rod to one which can lift & carry more line in the air.
My experience with light DT lines was that they were very hard to speycast in any appreciable wind, if the loop was blown out of shape they just lacked the mass to break the surface tension & achieve turn over once you had more than about 60 feet out, but on a calm day they gave a lovely presentation.
Regards, Tyke.