Have you finally come to any conclusions... or are there in fact missing fixes with Reforged in the last few patches?
It is not impossible that there is a possibility of some failure with the model itself, even if it is Low Quality. But in this case, precisely because OldCannon is an SD with few vertices, few anims, with little texture and low frame rates, everything seems strange to go wrong.
But within a general context, the units can have different types of impact, from the massive amount used on the same screen (which necessarily relates to the hardware, depending on the demand on HD quality or graphics effects you impose) or issues of mechanical limitations of Wc3 itself, it is not very easy to identify exactly what causes drops in FPS.
Personally, I don't think there can be any big problems even if you zoom in and see a fight with thousands of units clashing in Wc3. As long as you have good hardware and are aware of the game engine limits required by Wc3, because knowing how to deal with it, using "juggles" that deceive the limitations of Wc3, you won't have (or shouldn't) have any kind of problem.
WC3 MECHANICS LIMITATIONS: I will enumerate some main ones related to units, examples of collision pathing that limits a number of units for each Player (this does not necessarily cause FPS drops, but only limitation on movement for significant amounts of units), amounts of units used in relation to amounts of "Playable Players" on the map, many are unaware or little refer this limitation, but it exists, if you really want to have a few thousand units without having any evidence of Lag during big combats, insert more units for Players "unplayable" and force them to be allies and to move whenever necessary to the positions of their "playable" units. Unit edits in OE, using large amount of units to combat at the same time, having high attack speed and still being area of effect to reach large dimensions, if there are many units within this area of influence, it may cause some delay , although not as abundant. Another big or main villain is Triggers (such as Spawns of large amounts of units having the same point as source), but if you tested each one separately or removed all triggers, it can't be.
GRAPHIC/MODELS LIMITATIONS (SD, HD): I hope it's not a problem with the latest Reforged patches. I'm sure in this case it could be some modeling problem or even some corrupted track (3D experts can find this), you use SD and shouldn't have problems even with large amount of units, to be more specific, not even highpolis I believe to be a problem for the performance of the game, but the abundant amounts of frames per second in each animation, I think this is the great perpetrator of the FPS drop problems for most maps that use many custom models in this condition and that regardless of whether they are in SD or HD format, but even if you use many models on the same screen with a high amount of frames, this can be solved with a high-end computer... Note: i never noticed any interference with the game's FPS flow, even when using an abundant number of highpolis units, please if anyone disagrees, prove me know otherwise.
From the experience I had with HD use, I strongly believe that models with a high amount of polygons do not cause lags, they can cause visual discomfort or graphical failures if your hardware is very bad, this story of problems with highpolis really seems to be a myth, they can be a problem in using a large amount on the same screen for the Warcraft Classic engine, I'm not sure about the unit limit, but unlike that with Warcraft Reforged, it supports as many high polygon units as you want, as long as that you have a good Video Card. Using high resolution textures, I'm not sure, but it might be a problem for lower hardware, but if they're within the classic standard and less than 512 pixels, it shouldn't give you any problems. Now, high textures and abusing the graphic effects for lighting and shadows of the new engine, be ready to have a good computer.