Telegrams were much shorter than letters but I don't understand your question about the difficulty of spelling out each letter. The difference was caused by the cost of sending them.

For a letter the sender pays a standard amount for the letter partly depending on distance, i.e. Local versus international, and sometimes by weight, e.g. for airmail. The longer the letter the more value you get for your postage.

A telegram was written out on a standard form, and handed in at the Post Office/telegraph office with payment, but you paid by the word. It was dearer than a letter to start with and the longer it was, the more it cost. Brevity saved money and the fewer words used the better so "telegraphese" was terse and omitted any word not deemed absolutely necessary for understanding the message.

Difficulty in spelling wasn't the issue as the telegram was processed by professional staff. It wasn't like a tweet where the limit was on number of letters. The payment was per word so words were spelled out in full with as few words as possible.