Some of my family & friends had 'slow texts' and we tried every fix under the sun.
One of my mobile network techies put a very interesting spin on the isue, so here it is:-
Twas nothing to do with our phones or the recipients phones.
It was something known as the "Warrington Switch" 
That is to say there was a hold up/delay in the text [if I can put it this way] being relayed from sender to recipient via a bit of kit which was [is?] to be found in Warrington.
The issue was random and went away as quickly as it came.
I'd keep a date/time log of when this is happening and then contact bt mobile [and or the provider of the recipient of your messages]. Theres lots of fun to be had in conducting experiments with texts and keeping a log; this may be nothing to do with your issue but I'm throwing it into the pot as I wish I had known of the existence of the "Warrington Switch" which resonates with "the Pendle witch" etc.
Good luck and let us know the outcome.
