welcome to the forum,
can you post the output of rclone config minio: and rclone version ?
really hard to generalize a set of optimized flags.
much depends on internet connection, latency and your codec, applications using the codec, and so many other unkown factors not mentioned.
rclone mount simply random-access reads chunks of a file, as requested by an application.
then, if using the optional vfs file cache, saves the chunks for a period of time.
it is the your application that will control the prefetch before/while issue.
depends on how you define optimal ?
imho, create a performance test for the codec, run rclone mount with default values, establish baseline performance.
based on that, we can see what, if anything, needs to be tweaked.
and can check out my summary of the two types of vfs cache