2. Transactions are visible, only information like total amount are flickering, but after some time it is visible
To get total count and so on a lot of api requests are necessary. It depends on the speed of your node how fast all information are available.
3. I think this is main problem after enable it everything is fine. Why not enabled by default?
Security reasons, please have a look at plugin section in the changelog:
https://github.com/NeXTHorizon/hz-source/blob/master/changelogs/nxt-client-1.5.0e.changelog.txt#L214-L236