I have personally posted many times on a number of forums that Swiftkey can exhibit lag. It depends quite a bit on usage patterns.
For instance, I have installed SK on every device I've owned since SK dropped, and when editing docs more than a page/two, it's not just lag, it crawls to a complete stop. SK word prediction is excellent, but I could never use SK because of this one fatal flaw for my usage pattern.
Turns out that over the years, many SK users have complained of lag and when working with docs more than a page, lag so bad it grinds to a halt. SK has never dealt with this issue, despite the requests to do so.
I use Google keyboard. It's excellent IMO.