I have encountered a similar issue, but it happened to me with SW4STM32 v1.
It has been a while since I looked into the issue and found a solution online for it - it has something to do with the build logs that Eclipse maintains - when they get too big, build speed drops dramatically. The fix involved something like turning build logging off, along with deleting the log file. I wish I could remember the details of just what settings you had to change and which file(s) to delete. I read about how to do it on some (other) forum like Stack Overflow when I was researching the problem over a year ago. It’s more of an Eclipse bug or shortcoming, it’s not something (I think) that is unique to the Ac6’s SW4STM32 plugins.
An alternative to this fix that I discovered on my own is to change the project properties to use the “internal” builder. Do this opening a project’s properties dialog (select project root in Project Explorer pane and hit Alt-Enter), select “C/C++ Build” from the lefthand list, then select “Internal builder” from the /Builder type/ dropdown in the right dialog. Doing this will speed up the build considerably (even if you’re not experiencing the several-seconds-per-file build speed slowdown), and will produce considerably less clutter in the build console output for each source file processed.