I think performance, besides the aesthetics, have always been the Achiles' heel of Swiftkey.
I used Gboard for a few weeks and I when I returned to Swiftkey I was deeply disturbed by the UX.
I made some GIFs to show what I mean:
1 - The animations aren't smooth. On the contrary, they're abrupt.
- Gboard (https://i.imgur.com/jEcglaQ.gif)
- Swiftkey (https://i.imgur.com/UtsBKah.gif)
2 - The key pop up feature is inconsistent. Notice how Gboard visually registers all the inputs, regardless of how fast you type. Swiftkey fails to exhibit the pop ups when the keys are being pressed rapidly.
- Gboard (https://i.imgur.com/LM6bSrS.gif)
- Swiftkey (https://i.imgur.com/uNwXHbk.gif)
There are more issues, but these two are the responsible for the bad impression of the app in the every day usage.