I don't fully understand this - It isn't critical to virus scan files when reading or writing them. It's important to scan them between a write and a read.

That means many written files (which won't be read for a long time) can be scanned hours later.

Others can be checked for matching a "known good" hash whitelist which can be downloaded from the net. That should cover all kinds of files which are parts of software distributions etc.

That leaves very few files which really need real time scanning.