That’s a feature, not a bug. 
It’s an anonymity feature which is creating this issue.
The context is:
The the defense is:
Keystroke Deanonymization - Whonix chapter Kloak in Whonix wiki
Kloak is the cause of the observed behavior.
Can be recognized as a bug. But as a cannot fix or patches required.
(Bug Reports, Software Development and Feature Requests chapter Contributions in Whonix wiki)
Ideally we would have keystroke anonymization using Kloak and at the same time no user visible delays while typing but that seems unlikely given the current support status of kloak upstream.