There are a few possible reasons for this problem.
1. At the time the texts were sent, your sending phone was not connected to the network (out of range of the nearest tower or you were, as you say, in signal shadow area). What will happen here is that your phone will keep retrying whenever it gets a connection to the network, but if the signal level is low or intermittent, the text might not go straight away. Your phone generally will
not notify you if this is happening, and the delays caused by this should not be long.
If you were somewhere near a radio or a stereo at the time, you might hear this sound coming from the speakers
This is RF interference caused by the pulse transmitter in your phone sending interrogation pulses as it tries to find a network to connect to.
2. The second problem could be with the receiving phone. If it is out of the range of a tower or in a signal shadow area, the network cannot deliver the text. The text will be held until such time as the network detects that the receiving phone is connected again. Delays for this can be considerably longer in periods of high traffic and your message may not be immediately ready for delivery at the time you connect.
3. Network overload. At time of high network traffic, there could be considerable delays while your message is waiting in the queue for sending or delivery.
I don't believe the length if the text (number of words) will have any noticeable impact on delivery times, although a PXT will likely take longer to send (upload) and receive (download).
SIDE NOTE: It is these pulse transmissions that aviation authorities worry about with regard to having your cellphone switched on in flight. If your phone is on, it will be continually trying to connect to a network and transmitting these pulses. IIRC from my radar days, pulses have a very wide bandwidth, and can create harmonic signal interference well outside the frequency band in which they are transmitted. Many of the nav-aids on modern airliners such as VOR, ILS and IFF (squawk box) use pulse transmissions over similar and/or adjacent RF bands to those used by cellphones. While your phone alone might not cause a problem, you multiply that by a coupe of hundred phones on the plane all transmitting pulses, and the potential for interference increases significantly