In the course of testing the other v12.6 bug I just reported (system freeze due to scanning a BootCamp partition with Compressed Files ON), I noted another problem.
Evidently the new v12.6 scan engine is creating a baziillion small files -- it appears it is creating one for each file it scans. Presumably this is in aid of speeding up scans -- i.e, the engine can detect a file has been previously scanned and has not changed since.
The PROBLEM is that these files are created in such a way that Time Machine is now obliged to back them up!
So after a System Scan or an Idle Scan, the next Time Machine incremental backup takes *FOREVER*, even though the Mac has been idle and so there's not really any significant amount of "real" file changes needing backup.
I believe this is new behavior in v12.6.
This is on a MacBook Pro 17" (mid 2010), running OS X 10.8.5, fully updated, with 4GB memory.
--Bob