These links will not help you per se, but you can see that other people are facing this issue as well. The solution probably lies in the next SFOS upgrade. I hope it gets fixed then.
I think the issue is due to something in Sailfish 4.3.
I updated my 10II to 4.3 and Viber is completely unusable, where on my XA2 running 4.2 its running with no problems what so ever.
REPRODUCIBILITY (% or how often): 100%
BUILD ID = OS VERSION (Settings > About product): 4.3.0.12
HARDWARE (XA2, X10, X10 II, …): XA2 Plus
UI LANGUAGE: English
REGRESSION: (compared to previous public release: Yes, No, ?): yes