First you must imagine. Imagine that his device is not your device. Imagine that it is slower. Imagine it is slow enough to pass his threshold of unusable.
It started to be very slow some time ago for me, on my Android (Xiaomi Redmi 6 Pro). I'm thinking about not continuing my subscription.
I've attributed this to the increased number of notes, but after reading the article, I'm starting to think it might be because of the programming decisions, at least partially
I am not sure how the app is unusable for you. I use Standard Notes on my iPhone without any problems.