I have been a Swiftkey owner since the earliest days. A number of years ago, I realized that it doesn't work with documents over a page or two. If all you do is text a couple of sentences at a time, SK is tops. If you need to do more than that, it's broken. Technically, you can call it lag, but once the response time for a keypress is over 10 seconds in duration, I call it broken. This is the case across Android versions, across phones, across word processing apps, across the years. It's thoroughly documented and a commonly-known issue with SK.
Once a year I go on their VIP forum and bring up this issue - nothing is ever done to fix it. I suspect it's an inherent flaw in their app design. How can a product that is so good on one hand, suck so bad on the other?
Since I often work in docs that are more than a page or two long, I use Google keyboard, and have for years now. I prefer the prediction of SK, but hey, it doesn't work. I've also used most of the other leading keyboards and I find the stock Samsung keyboard to be quite good. But, Google keyboard is reliable, accurate, well-designed and fast in docs over a page in length.