The usual causes of slow start on the diesel are fuel hoses more than a few years old that have developed microscopic pores and are allowing air infiltration (they will not be wet from diesel, but still leak air), the glow plugs or glow plug relay are failing, or a clogged fuel filter.
Fuel hoses should be changed every three to five years as part of regular maintenance if you live in a normal climate. In Texas and hot areas it will be necessary to change them every two years. A good test to see if the hoses are the problem is to reach in and prime the fuel system before you start the engine. Primer is the button on top of the fuel filter. If it starts immediately like it should after priming, then you know you have hose issues.
The glow plug relay is easy to test by using a multimeter on the 12 volt voltmeter setting, on the contact bar across the top of the glow plugs when you first turn on the key. If you have power that cycles on, then shuts off after a very short time, the relay is good.
The glow plugs themselves are easy to test with the ohm setting on the multimeter, but does require removing the contact bar and testing each one individually. If they have continuity they are generally good.
Fuel filter is a bit of a no-brainer, as is the drain on the bottom of it being a bit loose. But, there is a potential for air leakage on the priming pump on top of the filter. Usually they last for many years, but carefully inspect it for torn cover or looseness.
