HPS takes time to light up

Is it dim or is it not turning on at all? You dont have a spare MH or HPS to test out?

 

check the bulb if you have black soot at each end even one end of the salty rod(in the bulb) its too worn and replace the bulb,
is the ballast hot ? not warm, is normal or any oil sticky stuff leaked out replace the ballast.

we need pics for more,

until then whip up an array of cfl's or home type led bulbs, to get you by

good luck

 

Sounds like your bulb might have a failed capacitor.
Correcting Problems With HPS Lamps - Greenhouse Product News

Failed Capacitor
The second most common failure is the capacitor. If screwing in a new bulb does not solve the problem, the lamp must be taken down and disassembled. If the capacitor is enlarged, it has failed and needs replacement. Capacitors also can partially fail. Here, the fixture starts and runs continuously but comes up to only 10-60 percent of its maximum brightness. HPS fixtures with a partially failed capacitor have a lower light output, a lower power factor and reduced efficiency. Five to 10 percent of the HPS lamps we've purchased in the past 25 years have had partially failed capacitors. This partial failure is not always visually apparent, and lights continue to function at reduced efficiency for extended periods of time

You might have an old bulb also.

End of life
At the end of life, high-pressure sodium lamps exhibit a phenomenon known as cycling, which is caused by a loss of sodium in the arc. Sodium is a highly reactive element, and is easily lost by combination with other elements, and migration through the arc tube walls. As a result, these lamps can be started at a relatively low voltage but as they heat up during operation, the internal gas pressure within the arc tube rises and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for the arc eventually rises to exceed the maximum voltage output by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly.
More sophisticated ballast designs detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts.
LPS lamp failure does not result in cycling; rather, the lamp will simply not strike, and will maintain its dull red glow exhibited during the start up phase.