cp performance issue

As is observed in https://github.com/nushell/nushell/issues/14778, uu_cp seems to have performance issues when dealing with large directories.

> timeit ~/.cargo/bin/coreutils cp cymbol_out/ -r cymbol_out_bak
52sec 95ms 4µs 960ns
> timeit ~/.cargo/bin/coreutils rm -rf cymbol_out_bak
10ms 929µs 36ns
> ~/.cargo/bin/coreutils -h
coreutils 0.0.28 (multi-call binary)
...

In my case, the directory is of ~2.5GB containing a list of big json files. GNU coreutils cp takes <3s to complete the operation but uu_cp takes ~50s. I've also tried on zsh with similar results.

> time ~/.cargo/bin/coreutils cp cymbol_out -r cymbol_out_bak
real    50.43s
user    0.00s
sys     3.23s
> time ~/.cargo/bin/coreutils rm -r cymbol_out_bak
real    0.56s
user    0.00s
sys     0.56s
> time cp cymbol_out -r cymbol_out_bak
real    2.81s
user    0.00s
sys     2.81s
> time rm -r cymbol_out_bak
real    0.54s
user    0.00s
sys     0.54s

Start by reproducing the reported uu_cp versus GNU cp timings with the large directory and JSON files described in the issue. Compare recursive-copy behavior and measure where the time is spent; done means uu_cp no longer shows the reported large-directory slowdown and the benchmark confirms the improvement.