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Warning - this is a hack that I pretty much stumbled onto. It isn't stable, it can affect the game in undesirable ways, and I haven't tested it beyond the first mission, and I don't know the full extent of its side effects. But from what I tested, this is a playable workaround.

Because of all this, I only recommend this to experienced DOSBox users. But I would value any feedback from anyone who can test this, or has some insight. Maybe this technique is already known, but I didn't read about it anywhere.

So first, start a new game, and save it right away. Then quit.

Now, crank up your DOSBox cycles in the config. I went with 20000, but this was a blind test, and maybe other values are more ideal. Experienced DOSBox users should know how to do this.

Go to the WC1 directory. Open the folder "GAMEDAT." Find the file "SAVEGAME.WLD" and make a copy of it. Move the copy to another directory. Rename it "CRUSADE.WLD." Now move the file back to GAMEDAT, overwriting the existing "CRUSADE.WLD" file.

Essentially what you are doing here is renaming your save file so that Secret Missions 2 can find it.

Now, launch The Secret Missions 2: Crusade. GOG by default puts a shortcut to it in your start menu.

Load a saved game. You should see the saved game you just made earlier. Load it, and you should be able to play missions from the Vega campaign, but in SM2.EXE.

When I played this way, the lag was gone, and the gameplay not too fast either.

Some things I did notice:
* The first mission seems much harder than I remember it. This could be faulty memory, but I get the impression that enemy ships are tougher and more evasive.
* Laser battery charging is odd. Outside combat, I could fire indefinitely and not run out. In combat, sustained fire drains the battery quickly. I don't know which one is normal.
* Cut-scenes are crazy fast. So is auto-pilot.
Adding to the OP, from a gamefaqs guide:

9.2 Run WC missions with SM2

As this modification involves some unauthorized file
manipulations, you perform these at your own risk.

Start WC, and start a new campaign. Then visit the Barracks and
save the file. Remember this save’s name. Exit the game.

If you have SM2 installed, there should be a CRUSADE.WLD in the
GAMEDAT subdirectory in the directory where WC and SM’s are
installed. Make a copy of that file.

There should be a SAVEGAME.WLD in the GAMEDAT directory, created
by WC. Copy it to CURSADE.WLD in the same directory.

Now you can start SM2. Continue existing game, and pick that save
name you saved in WC. Now you can play WC missions! You get frame
rate adjustments, and some of the missions behave "better" with
SM2.

There are a few problems, of course. 1) Paladin will show up, but
he’ll be referred to as Doomsday. Bossman will be referred to as
Jazz. 2) Dralthi will be Mk2’s (mass driver instead of laser). 3)
Instead of different behavior for different aces, they’ll all now
fly like Drakhai and be named Drakhai instead.