Does the tractor have an indicator light for the glow plugs? My Massey Ferguson has a little coil that slowly turns bright red as you're holding the glow plugs on that gives you a relative idea of when they're ready to start. It's not uncommon for me to give it a good 15 seconds, even more on super super cold winter days. Try giving it 1/3 or 1/2 throttle too, that should help engine speed stabilize faster when it first comes to life.
I also have a diesel Volkswagen that's 15 years old now and almost 300,000 miles. That starting system is more advanced in that the glow plugs stay on automatically for a set amount of time determined by the computer based on ambient temperature. The dash has a light that goes out when they're ready.
Assuming youre system is working correctly. But for a quick check if you have a voltmeter, set it to ohms, pop the wire off each glow plug, hold one end to the metal tip of each plug, and the other end to ground. Resistance should be a couple of ohms. When the plugs go bad they will go to infinity, or I've seen partially failed plugs that will read several hundred ohms.
I will say on my Massey starting cold is much more ugly than my Volkswagen, it starts right up no problem but there's a lot of smoke, and the engine turns very slow for several seconds (slower than idle) until the engine can generate enough heat to stabilize.
