I did not see anything "special" about TRIM and Samsung 840 Pro and 850 Pro SSDs (I have such models). In my research, most of the articles state that TRIM is beneficial for prolonging the life of SSDs. Yet, I did see where enabling TRIM does cause some slowness with High Sierra (would be fantastic if Mojave makes any improvement on that). In my case, my processing needs are not that intensive, especially on my MacBook Air. I do have TRIM turned off on my Mac Mini (with a Samsung 840 Pro 256 gig SSD), along with having plenty of free space on that SSD (and of course my efforts in keeping my Macs "lean, mean, and clean").
I can't specifically address the speed increase issue as far as High Sierra v. Mojave are concerned. I think my feelings about HS have been made pretty clear before. I think it's a cat turd rolled in cracker crumbs.
Then again, I kept my experience with HS quite limited. I installed it only on an external drive on a USB 3 bus and later put it in a Thunderbolt enclosure to see if that helped speed it up. It didn't. At least by any appreciable measure from my perspective.
Aside from the early security issues, which I was never personally satisfied about being really resolved, HS simply never demonstrated itself as being as stable as either of its last two OS predecessors. That said, I think Sierra was absolutely outstanding from a stability perspective. And it was/is plenty fast.
HS was like the geriatric customer in front of you at Krogers who insists on paying exact change, can't find their change purse, counts out one bill and coin at a time, has to restart counting a couple of times, then says, "hell with it, I'll write a check," then has to fish around for what seems like an eternity to find the checkbook, can't find a pen, then can't find a pen that works... You get my point. Of course this was performing tasks with Premiere that are often time consuming in the first place.
All the rocks I've thrown at High Sierra are cast based purely on my own personal experience with it. Everyone's systems are unique unto themselves, and as the old saying goes, "your mileage may vary."
With Mojave on my MBP, I have seen a nice little performance boost going directly there from Sierra 10.12.6. Other than having to do a little dance with Terminal to get Adobe CS stuff installed and working properly, I've had nary a hiccup. But I had to deal with Terminal with Sierra to get the Adobe stuff to work as well, so I suppose that doesn't count.
As a matter of fact, I can boot from either of the two clones I keep of my MBP's system, and they perform quite nicely, even though both are 7,200 rpm HDDs and are connected via USB 3. They take perhaps 10-15 seconds longer to boot, but I don't think that's shaggy at all.
Don't quote me, but I believe I read somewhere on OWC's site that the Fusion drive issue with Mojave has been ironed out. It didn't stick too tightly in my brain because I don't have any fusion drives.
This entire Mojave business has me looking at myself a bit more closely in the mirror lately. I keep asking, "who the hell are you and where are you holding Allen captive, no doubt bound and gagged?"
I'm NOT an early adopter. At least I never had been before, going back to the days of System 7. I made an exception the day iOS 12 was released because iOS 11.4 was another cracker crumb-encrusted cat turd and I couldn't wait to get rid of that turkey. After seeing how great iOS 12 was, I downloaded Mojave, but mulled it over a few days before I installed in on an external drive. When I saw how well it seemed to work, I took the plunge and installed it on my MBP, which is my workhorse machine. And so far, I've not regretted it for a second.
On the TRIM thing, I'm persuaded it's a good thing to turn on. I've often scratched my head as to why Apple would not automatically enable TRIM when installing a third-party SSD. I didn't see any slowdown on any of my SSDs.