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Syndicate Plus running under DOSBox is unplayable IMO.

I can never get the correct speed. If its too fast the game starts behaving strangely. Enemies know your position too quickly, country happiness level and research is erratic. Too slow and it takes an age for your agents to walk through a level.

Do yourself a favor and install PCem. Its a machine emulator kinda like VM, but not just DOS and therefore runs at realistic speeds. You can set up 386, 486 and Pentium machines.

Its not straightforward to setup though and if you've never actually owned an old DOS based PC you may find some bit challenging. You have to setup things right from BIOS, the mouse drive all the way to XMS/EMS! Boot disks! Remember those days?

Here's a decent tutorial to get you configured -

I set up a Pentium 133 with DOS 6.22 and Syndicate runs great now!

You'll need to extract the game files from the GOG installer using a tool like innoextract.