The amlogic chipset used on the original t8s (one of the s8xx series) is not capable of being successfully updated to android 5. I run an older t8 on android 4.4.2. Using dbmc 16.3. It is definitely not slow so there is some other cause affecting your machine. Updating Kodi would definitely not cure a speed problem. If you would like to elaborate on "slow" including results of speed tests, confirming whether you are using wifi or Ethernet, whether android apps like YouTube experience problems etc, there may be other solutions.