It is difficult to directly compare 4.8 and 5.0 scans since you don't say what scan you did on 4.8. For me the scans on 5.0 after a few are quicker for a comparable 4.8 scan. Not to mention you can have greater customization of the 5.0 scans or create a custom scan.
However, initial scans in 5.0 are going to be slower as it is also populating the persistent cache database, which should after a while increase overall scan speed. I have never held much store in progress meters though mine appears to be working, but there is a lag, just started a scan whilst typing this and for a short while it showed 0% yet the other data showed it had scanned 17,750 files, 2.46GB of data in 24 seconds. There isn't a great deal of data on my system, no media files, etc. so perhaps on your system it is in a folder or partition that has very large files ?
So whilst it is showing 0% scanned on yours is it also reporting what has been scanned ?
Use persistent caching - if persistent caching is used, the information about
the scanned file is stored in the permanent memory. This means it is not lost
after a system restart and it is also not affected by virus definition updates.
Consequently, persistent caching is suitable only for files which are guaranteed
not to contain any virus infection e.g. operating system files, files signed by
trusted publishers, or other files covered by the avast! whitelist. This box is
checked by default; if you want all files to be scanned regardless of their
trust status, this box should be unchecked.
