Current fish author here. Thanks for giving fish a try! I hope you tested a 2.0 branch build, and not the 1.x train from 2009 - a lot has changed since then, including major performance improvements. Check $FISH_VERSION if you are unsure.

'ls' is slower on fish because it passes the -G ("colorize”) flag to /bin/ls, which gives you nice colors, but requires a stat of each file to determine its type. If you want faster but uncolored output, you could redefine the ls function to invoke the ls command with no arguments:

    alias ls 'command ls'

For other instances of slowness or bad completions, we’d sure appreciate it if you opened an issue on the github page at http://github.com/fish-shell/fish-shell/issues/