While still mainly a Windows user, I've tinkered around with various OS'es over the years.
I also tinker with old hardware a lot, and will often slap a Linux distro on an old, rebuilt PC or old, fixed up laptop.
My first go at Zorin was a while back, I liked it but at that time it wasn't something that I would be using daily (some things were still missing for me, a few other things were still a bit too reliant on opening up a console and messing about with commands to do things that should've been set up already, stuff like that).
Still, I liked it enough that Zorin remained on my list of OS'es to install on old hardware.
I've also occasionally installed it next to Windows on my regular, more modern PC - and if you think Zorin works fast on older hardware, oh boy, on a beefier current-day machine built for gaming/heavy graphics, it'll drag the roadrunner into a dark alley, mug that long-legged chicken and be out of there before the roadrunner even realizes it just got mugged.
Currently I installed the core edition next to Windows on an old laptop (12-ish years old) that a friend brought in for repairs, and despite the laptop only having very underwhelming onboard graphics, a CPU that might've been among the first of AMD's 64-bit CPU's, and only having 4GB RAM... Zorin still runs quite smooth on it.
My only gripes so far are that the menu button (think of Windows' "start" button) sometimes seems to just vanish (I suspect it simply goes too far to the left for some reason, way outside the screen's range) and that I've yet to find a way to port a self-made desktop theme which I created on another Linux OS years ago.
Oh, and the keyboard layout... I've found one that's "close enough" to what this laptop has, leaving just a few minor inconveniences regarding 3-5 missing keys that aren't even used regularly... but I'm putting that particular issue down to the laptop having some obscure proprietary layout (at least, I vaguely recall the manufacturer often trying to make everything about their products "unique" (translation: "Good luck trying to plug any standard cables made by anyone else into any of our laptops").
So, long story short, after having seen Zorin Core 15.3 do just fine on this antiquated laptop, it looks like I'll be making Zorin into a permanent resident on my regular desktop PC (dual-boot), maybe even upgrading to ultimate when the bills that will be draining my bank account for the next few months (due to a hospital stay and spending close to 2 months in a recovery "hotel" after surgery) no longer wipe my bank account out.
I make my own themes as well. Maybe we can look over yours and see where it is stalling out.
As an aside, many of us testing the current Alpha build of Zorin OS 16 have found it to be Blazing Fast in speeds. This is partly due to ZorinGroups backporting gnome-shell. But possibly duie to other attributes.
While the machine I am using is not old, the experience of Zorin OS 16 in testing has been a true phenomenon in speeds, beating out Ubuntu 20.04 and Ubuntu 21.04 on my machine Hands down. Or as you put it, enough to give the Roadrunner jetlag from the mugging.
That theme was made for an older KDE desktop, with a bit of help from a Windows-based live desktop maker ("Gritly" or "Grittly", one of those was the correct spelling but Google hasn't been much help in finding it back).
I know my way around the different desktops available under varying distro's, but not well enough for me to port things it seems.
So, thanks, I just might take you up on that.
And yeah, the speed definitely impresses, I've only seen one or two other distro's that might have comparable speed (from the ones that I've messed with... there are still plenty of Linux distro's and other OS'es that I've yet to put my hands on) but with Zorin it's the combination of speed and "works out of the box" that hits me in the head.
Ubuntu... I think my last installed version of "plain" Ubuntu was 10.0x (Lucid Lynx) so I can't comment on that exact comparison, but I did try out a few Ubuntu-based distro's within the last year which are probably in the same ballpark as 20.04 for speed, and Zorin definitely has those beat.
My desktop, with 16GB RAM, dedicated GPU with 8GB RAM, a Ryzen 7, and an NVMe SSD, will probably have Zorin running at speeds that I've never even thought possible before.
And that menu button vanishing, Iḿ already a step closer on figuring out what might be causing that. It's easy enough to bring the button back by briefly switching the theme/desktop layout and then again switching back, and it only seems to occur when the "activities" button is active... and I'm already a few levels deep in google searches that might help me track down what I should be looking at.
Ah, I see. Yes, KDE - QT based. Zorin uses gtk3. How are you with css?
If you like you can start a dedicated thread on that topic where we can work on a port or you can shoot me a PM.
CSS Iḿ familiar with, I have gotten into CSS plenty of times building websites, and years ago I even briefly held a job as an actual webdesigner... though I'll have to admit that there are still things that I have to look up (google and a collection of books on webdesign have always been part of my arsenal in that regard).
QT... maybe I need to slap myself on the head, my mind never made that link with KDE for my theme; between 2002 and 2004 I used to make skins for a QT-based media player.
Even then, I still relied heavily on keeping notes in textfiles and occasionally on help from a few friends on IRC if I wanted to make something more advanced, but at the very least I have an understanding
of it.
I'll make a thread on it later on.
The menu button, looks like you're right; one of the search results that came up took me to a thread on reddit regarding a similar issue, and a link in one of the posts there took me to an older post on stackexchange... which had links to other places, but each of those threads/posts also seemed to point at two possible things - one being about native host connector, the other about something not playing nice if font sizes are altered (which I've done when I first noticed the button vanishing). I'm not yet clear on how that relates to it only happening when the activities button is active, but it's a trail that I can follow, so I'll find out eventually.