Along with other factors like fuel mix and compression, the glow plug can be used to adjust timing in your engine. Generally, the hotter the fuel used, the cooler the glow plug should be. Conversely, with a low nitro fuel you generally use a hot plug.
One symptom of a mismatch is, with the needle set properly, when you remove the booster battery, the engine slows down slightly. To compensate you can: Up the nitro, up the compression (adjust head shims), or use a hotter plug.
Try the plug first because this is also a symptom when your plug is becoming fouled with a coating of something. If you can't flush it off, you have to discard the plug.
In winter some flyers up the nitro a bit because they are already using a hot plug.
Get to know your plugs well and you may be flying while others are fiddling with their starter (or flipping).

George