I open Duckduckgo and Google in Firefox on my Android 5.1 phone. Usually I have multiple tabs open, but not that many. I start typing, and after two or three characters, the typing freezes. I can continue to type, but nothing is displayed. Then after a while, it catches up and then I can continue to work normally. Then, if I do another search, this doesn't happen anymore. It seems to happen mostly after not using Firefox for a while.
What is causing this behaviour? Is it the keyboard or Firefox? In other apps I don't have problems with the keyboard. The keyboard is Xperia from Sony.