I am truly confused with this sentence you wrote: "(it is convenient, for examle, while chatting, to save time)?". Also, you have me totally lost by this "informal speech" you referred to. Saying "ευχαριστώ" is for informal and formal speech alike. What is it you meant??? But regarding "ευχαριστώ", yes, there is a way to reduce its use by saying "να είσαι, να'σαι/να είστε/να'στε καλά."