Well I am going to share my experience with swiftkeyboard. I am using REDMI 3S PRIME phone currently so when I got my phone it was pre-installed app.
I stared using app and found it has many features like Hinglish language which support Indian Hindi English language and I set Key board on autocorrection and suggesting words option. So when I type any Hindi English word wrongly it auto correct it. In Hindi English other keyboard just support English autocorrection so if you say kya it will auto correct your work with similar English work. But if you use swiftkeyboard and type ’kay’ it may be Autocorrect work ’kya’.
Second thing you get lots of Theme in it and you can download different theme too.
Third this is very interesting in it. It is resize like if your eyesight is low they you can use max size. Well it helps to resize your keyboard.
Forth this is gesture but most of keyboard support gesture.
I never found any lagg in this keyboard it is very easy and low memory consuming app.