should be again related to OS, just did a full scan and quick scan, both with persistence cache (using and writing), times are as usual, progress bar behavior is normal. On the other hand, I scanned a big *.rar archive, no progress bar shown at all until the scan is over, not even a 100% one. I'm not surprised, although all files inside the archive are scanned (shown in the dialog box), it's still interpreted somehow as just one file by Avast.
edit: OK I see some have this issue on Win7 too, but what can I say, I don't have that bug. I'll check later on another system that has the "free version" installed.