Check the "Scan count" for the Standard Shield provider? How much does it show? What file is being scanned all the time (see the "Last scanned" entry)
(double-click the avast tray icon to access these things)
Moving the sensivity slider to High basically instructs avast to scan ALL files on change. This can be quite a BIG overhead e.g. in case of log or ini files, which are usually written all the time.
Therefore, if you insist on leaving it on "High", it might be a good idea to put the most common benign files to the exception list (e.g. *.log, *.ini, *.txt etc).
Thanks
Vlk