--buffer-size uses RAM and --vfs-read-ahead uses disk. Only use one of them and leave the other default. Especially set --buffer-size not to high when use --vfs-read-ahead.
my advice:
use a high --buffer-size when you have enough RAM. Calculate with buffer-size * transfers for total RAM usage by rclone mount. If you haven't enough RAM use the disk to buffer. This can be set via --vfs-read-ahead. Notice that you need --vfs-cache-mode full to enable buffering to disk.
when Plex reads the file notice that the whole buffer (256M) per file are downloaded.
finally jump over to this thread and spend some time to understand how rclone works in interaction with Plex/Emby