The Amazon Fire Stick although a great device is now struggling with Kodi 17 with MANY users complaining of slowness, freezing and/or buffering playback.
Below is 3 steps to try to hopefully improve your playback.
FIRST GOTO SETTINGS / PLAYER SETTINGS / HIGHLIGHT VIDEOS AND ENABLE EXPERT MODE (as in the picture)
Then follow the below steps:
STEP 1: Go down to the processing section and Turn down Enable HQ Scalers to 0%
STEP 2: Disable MediaCodec (Surface)
STEP 3: REBOOT KODI AND TEST
If you still have minor issues you can try to disable Medic codec as well but try the (surface one first)
***VIDEO GUIDE BELOW***