What exactly does avconvert do with multiPass and disableFastStart parameters?

Solution 1:

Setting the -multiPass option means that the program tries to achieve a higher quality encoding by going through the whole movie more than once. Essentially you're making the trade off that you're getting a slightly higher quality output at the expense of taking considerably longer to encode.

Setting the -disableFastStart option means that the converted movie won't feature Fast Start. Fast Start means that you can place the movie on a web server and a user can start viewing the movie while downloading. If you disable Fast Start, the user will have to fully download the movie before it can be viewed. The upside is that fewer disk accesses (i.e. less "strain" on the system) is required when Fast Start is disabled.