I had a similar experience while working on a short story, using my iPhone 6+.
Even with the 6+'s larger screen, composition was still slower with the smaller touchscreen keyboard, more deliberate. The flow of ideas was more contemplative--more a process of thinking the sentence and then tapping it out, rather than having the thought and transcription more simultaneous.
Is the slower and more deliberate experience better or worse?
I'd say better or worse is the wrong evaluation rubric. The experience of writing was definitely different than traditional keyboarding, though. I am reminded of when Milton was blind, he would dictate verses of Paradise Lost to his daughter.
I believe it is possible to compose using a touchscreen keyboard. A more deliberate pace of composition is not bad; it might even result in the need for lesser revision. My experience was almost metacognitive--composing and being aware of the process of composing while in the experience.
I've written this contemplative piece about writing while using my laptop's keyboard, though. That frictionless flow of ideas from mind to fingertips to screen is awfully nice.
I wonder how it will be in the future when writers just speak to the computer. In the "old days," there were a few writers who dictated their work and then handed the recordings over to a typist to transcribe. And today, of course, we have the dictation software that is becoming more common. And those who have grown up with smartphones and touchscreens--prodigies who can touch-type with the phone in the pocket--I find that quite remarkable.
Has or does anyone else using the smartphone as a creative writing device? What has been your experience?