HTTP/2 was a very nice improvement, now we don't have the network overhead related to creating multiple connections to download multiple files, and is recommended to stop concatenating JS and CSS files to improve caching, but I'm not sure if from the perspective of I/O, having to read multiple files from hard drive will impact performance.
I don't know if Web servers like Apache or nginx have some RAM caching of frequent files and this avoids I/O overhead and I can stop concatenating and stop worrying about this.