If you have an unexpected a/c change that you find out about sometimes 20mins before landing, the crew has to close the bar (sometimes manually counting the whole thing plus cash), clean the aircraft after disembarkation then go to the other a/c and possibly clean that as well, all the while carrying out safety duties too. Try this in 20-30 mins and see how stressed you feel. Plus not all crew have a lot of experience in this, and if your purser is not a great organizer, th whole thing could end up in a huge chaos.
Try this in the heat of summer in say Alicante, without air conditioning (save fuel), in the middle of an 11 hour duty, looking forward to a nice minimum rest that has just been reduced by 2 hours due to the delay you had no control over, not even getting paid for the overtime and try to smile and be pleasant afterwards.
I agree tht professional atitude should be displayed at all times, but please understand that cabin crew are just regular humans, not super-humans. I, at least try my very best at all tmes, and if I do not provide excellent service on occasion, trust me it is not down to lack of my skills.
And if this airline involved is indeed a low-cost airline, I am afraid you might just get an overworked and "low-cost" cabin crew/service. You, as a "low-cost" airline pax did not pay enough for the crew to be looked after well, or at least some airline management likes to think so.