Helps to know year, make and model.

New glow plugs will help if there's something wrong with the old glow plugs. If your car uses loop type plugs, updating to pencil type plugs should reduce glow wait time though I don't know it helps starting. If your car uses pencil type plugs, the basic test is 0.7-0.8 ohms of resistance for each plug at the glow plug relay. If your car does not have the afterglow feature (standard in the US from 1990), defeating the engine start signal to the glow relay will allow glow to continue until the relay times out which should allow smoother running immediately after a cold start. Or you can upgrade to an afterglow relay.

Sixto
87 300D