rtfm is always good advice.
Universal diesels (I know, you don't have one) have glow plugs. The older model engines ran the (small) wiring from the batteries all the way to the cockpit panel and then back to the glow plugs. This required the 30 second warm up. Many of us have installed solenoids for the glow plug circuit which reduces the time to 15 seconds or less.
The newer Universal engines have a built in solenoid.
The newer Universals also have a (strange-to-me) start circuit wiring arrangement where the glow plugs are tied into the fuel pump and oil pressure switch requiring you to hold the glow plugs on to bleed the engine! Although the newer engines are self bleeding, it is rather a PITA. CRITICAL UPGRADES - DO THESE OR ELSE!!!
That said, the advice is to simply do what the manual tells you to do and then to completely understand how YOUR engine and wiring works. Beware PO changes, too.
In many cases, having the glow plugs ON at the same time you push the starter button or key switch will create difficulties because of the large electrical draw on the batteries. On our boat, I do the glow plugs, release them, then start.
Hard starting is usually related to bad electrical ground connections at the engine.