I can help verify... at least with my equipment that SLX loads considerably faster than Neolemmix.

I have every available pack installed in Neo, and I have converted every available pack for SLX and installed them there too.
My computer should be a pretty good test for this load-speed thing. However, I will say that Neo loads at an acceptable speed
for me. It probably takes 8-10 seconds, but after loading it's fine. SLX loads in about 2-3 seconds. I sometimes wish Neo could
share some of the level selector improvements.

https://u.pcloud.link/publink/show?code=XZVLiEVZYjLzkBRJmPybVkc6uxny8j7BpTe7

This is a level pack I put together in 2015 for Super Lemmini, updated for Super Lemmix.
This is 65 levels of Snow/xmas themed levels in four ranks made by members of the Lemmings community
long ago. I originally wanted to make this because I created the santa suit lemming animations for Super
Lemmini and wanted a level pack to come out at the same time.

Lots of thanks to Kaywhyn for converting the Super Lemmini pack so I could get this all started.

Cute 9 seems to work fine for me. saved 20 lemmings.

I love the YIPEE! sound when entering home. But I also get sick of hearing it so much I changed it to BOING!
It might be pleasant to mix the two... only yell yippee on first or last lemming? Or every 3rd one yells Yippee?
Or always do the "Boing" in ffwd mode?

I think three sound samples that have a random-fire method would be lifelike and not as robotic as a perfect 1-2-3 system.
A boing, a swoosh, and a yippee.

I attached the third swoosh sound I had in mind.

*Edit: I edited the WAV file and added a shortened version now.

Here's my notes on updating all four waters:
However, I updated the pack you're playing (World Tour) using the above method and there are a handful of levels that did not update.
Most did however. I love rock and roll, we're not going to take it, and more needed manually fixed. No idea why.
Here's the basic info...

Search and replace text:

STYLE orig_fire\r\n   PIECE water
to
STYLE orig_fire\r\n   PIECE lava

STYLE orig_marble\r\n   PIECE water
to
STYLE orig_marble\r\n   PIECE poison


STYLE ohno_bubble\r\n   PIECE water
to
STYLE ohno_bubble\r\n   PIECE blasticine

STYLE ohno_rock\r\n   PIECE water
to
STYLE ohno_rock\r\n   PIECE vinewater

That sounds good.
I'd personally be plenty happy with a simple version that adds a check-box to settings like (Faster FFWD Speed)
or something.
But fancier is okay too.

Here's a Logo file I made for this pack. I've made logos for every pack in my computer that was missing them. (That's how serious my life is lately)

Yes, I got the Milla graphics working.