Hi Anton,
1) Build 8116 seems to handle LSFS volumes better i.e. the sync seems to go faster, and garbage collection seems to work, too.
2) Areca has problems with SSDs and HDDs on the same controller. Once we removed the SSDs completely, performance of the HDD arrays improved.
With these 2 changes, shutdown takes about 2 mins, mounting all volumes ("creation") after a restart takes about 40 to 50 mins, resync afterwards takes about 30 to 40 mins. In total, a restart of both servers takes about 3 hours. That's certainly not superfast

but at least consistent, and we are happy that resync has finished after every restart since.