My Windows laptop (16 CPU cores and 16GB) gets 4 WU and starts crunching them. The first 95% are quite fast, but then the crunching slows down until it has processed 100% but just continues for hours. Sometimes a WU is processed, but most need to be aborted after 12 hours.

What do I need to change?

Theory tasks are a mixed bag of runtimes. 1 hour to 6days (even 10day tasks sometimes) is normal. No need to abort if the PC is running(The tasks do not survive a PC restart)

Boinc manager has no idea what is happening inside the virtualbox or docker container. So the % shown in the boinc manager is not relevant.

You may show the progress when using "Show Graphics" from BOINC Manager.
Highlight the running task and press the "Show Graphics" button. A local webpage will popup. Press the Logs link and then the running.log.
https://lhcathome.cern.ch/lhcathome/forum_thread.php?id=6438&postid=52989

Some tasks have a first step with "217 of 760 integrations done" or the second step that have all tasks that is counting up to "100k events processed'

Some may a take a whille. See thread below.
https://lhcathome.cern.ch/lhcathome/forum_thread.php?id=6436&postid=53008#53008