Slow?! Ahem...

Yes. Slow! (Be careful what you benchmark.):

C:\test>type junk99.pl
use Benchmark qw(cmpthese);

{
    package Foo1;
    sub new { bless $_[1], $_[0] }
    sub foo { $_[0]{foo} }
}

{
    package Foo2;
    use Moo;
    has foo => (is => 'ro');
}

our $foo1 = Foo1::->new({foo => 0});
our $foo2 = Foo2::->new({foo => 0});

cmpthese( -3, {
    plain_perl   => q[ $foo1->foo for 1 .. 1000; ],
    moo          => q[ $foo2->foo for 1 .. 1000; ],
});
C:\test>junk99
             Rate        moo plain_perl
moo        2065/s         --       -13%
plain_perl 2381/s        15%         --
[download]

(Think about it: Why would calling into C; unboxing perl vars to c vars ; and then calling the perl hash apis to access the hash fields (as C vars); and then wrapping those back into perl wrappers before returning; be quicker than a dedicated opcode -- written in C -- to access the hash?)