I have been riding my 1990 Serotta (56x55.5) and it has the SLX tubeset.

The geometry isn't really any "tighter" than my SL bikes, but I swear it has a pretty harsh (really stiff) feel. Granted it's geometry/tires/wheels/saddle etc. but it's the first lugged steel bike that has felt this way besides my steel track bikes. It's really noticeable.

I could have sworn that the word on the street of yesteryear was that SLX was for lighter riders, SL the standard, and SP for the big guys.

Anyway, the helical internal ribs on the SLX were designed for what end again?

Your answer is here: Frames and Framebuilding (1988) In Praise of Steel
Check page 112 in the Butting section.

It’s likely not the tubing. I have a SLX De Rosa and it’s not harsh riding at all.

I like bikes that can take a 28 c or a 32c tire. In terms of whether a bike rides “harsh” or not, I think tires matter more than the tubing.

I think it was for light riders because the really thin tubing might not hold up to a bigger/heavier rider. I don't know if that's the case.

SL, SP, SLX and SPX are all the same steel, or at least they were into the 1980s, maybe later. SLX is SL with added helical ribs at tube ends for added stiffness at the joints. SP is SL with thicker tube walls. SPX is SP plus the ribs. An SLX tubeset is heavier than an SL tubeset, an SPX set is heavier than an SP set.

If you have a Columbus -tubed frame from the 1960s up to some point in the 1970s, the sticker just says "Columbus," but it could have been the thinner/standard walled tubes (what came to be known as SL) or the thicker walled tubes (what came to be known as SP), depending on the builder's preferences for the particular frame.

SL tubed bikes with the early stamped front derailleur braze on mount, flexed a bunch. As a mechanic, I did not like it.
Soon enough, SLX assisted it taming the distortion.
And then the mounts evolved into investment cast, the attachment wrapping around the tube more.
Problem addressed.

Definitely a different lugged steel ride. I didn't have any preconceived notions with this frame, so the stiff and "dead" feel is an honest opinion. It rides very fast, and I am having a good time riding it, but boy, it has some unique qualities. I should track down the specified geometry.

My SOMEC in SL (56.5x54) with the ridiculously short wheelbase is less stiff by quite a bit.

Evidently Columbus actually had something going with the SLX/SPX tubesets besides a marketing gimmick.

It is possible that your down tube (and maybe even the seat tube and top tube) were SPX instead of SLX. SL wasn't all that stiff although both the down and top tube were .9/.6/.9. It isn't uncommon to have a slightly lighter top tube with other brands. A SLX frame alone (no fork) would weigh around 1900 grams + - (I'm going on memory) while SPX would be 2100 or 2200? That is one way to tell. A tubing sticker on the frame isn't always perfectly accurate.

It was my understanding that SLX "rifling" should help with BB flex (in theory anyway), but, since it didn't extend up the whole tube and wasn't found in all tubes, probably wouldn't make the ride feel more "harsh" by itself. I'm sure that others with more experience will explain why my understanding is completely incorrect.

What do you weigh? I'm 6' 2" 205 lbs. and SLX feels "springy" to me (in a good way!) - 57cm frame/700x23 tires/Illinois roads

I'm 191, and running Vittoria Corsa 700x25 @ 90 PSI like just about all my other vintage bikes. My 531 Paramount for example is night and day different with the same tires.

Pure opinion: for the most part, high end frames are made/assembled by humans, which means that each one is somewhat different. For example, have owned more than a dozen Merckx frames and only two of them really ‘spoke’ to me. Kept one and really regret selling the other. Same deal with other brands and models, just bought & sold one after the other until coming upon the right one that had a great ride for ME…which may not have the same ride qualities for YOU.

Truly mass produced frames are more consistent. For example, have owned at least a dozen 87 Schwinn Tempo’s over the years and each one has been very, very good. Good enought that someone always came along and offered to buy the one I had, and good enough that eventually I’d buy another one. Not high end, Columbus seamed tubing, but they just work really well. So, there’s that.

At my age there is little danger of seriously flexing a BB, but I must be insensitive as I have two Tommasini super prestige from the early '80s a SL and SLX and a '81 Pinarello Montello from the Levi Team SLX and had until recently a Pinarello Montello SL early 80's. While not perfectly similar builds I perceived no difference in frames, least of all the 1 oz+ difference in weight. As a keeper test I do run all bikes down a serious decline near the house at much higher speeds than I am happy with and hit some mild undulation near the bottom before a relatively sharp turn.

Investment cast lugs improved the frame stiffness compared to stamped?

yikes! that's surprising I always held Serotta frames in fairly high regard.

From my limited experience, the workmanship on the Serotta frame that I have was virtually flawless. The brazing and the mitering were second to none. I stripped the frame to bare metal to paint, so I really got to see everything. In comparison, the many (6+) Italian frames that I have stripped have always had cosmetic/brazing/alignment flaws.

Too many variables. A better Chance that investment cast lugs will result in no voids, but even investment cast lugs are tweaked from time to time.
Then what?
builder skill.