Fluorescents

They are not all the same. Don't buy cheap ones. Read web reviews of various lamps and choose the ones that get good reviews for brightening up time. There is a component called the 'ballast' which brings the lamp up when the switch is pressed. No gas-discharge lamp lights at full brilliance immediately. Better designs have more complex ballasts which reduces the time to full brilliance, but it cannot be eliminated altogether. Standard shapes usually brighten quicker than specialty ones.

Contrex-
Thank you.

Mine is a a straight line. Similar to what is used in many factories & offices.

With that kind of tube, the ballast is separate from the tube. Could be the ballast is failing. Also, I've seen some tubes headed towards failure that glow and flicker in the orange range before finally getting to normal performance.

Sorry I thought you meant a compact fluorescent lamp. An older type straight tube in good condition should light very quickly, so as Roger says, it's either the ballast or the tube. It's easier to check the tube first. Do you have a known good tube e.g. in another lamp? If the tube has discoloration inside the glass near the ends that usually means it is getting old.