assignment vs SELECT INTO

I found out today that direct assignment to a composite type is (at
least in my test) about 70% faster than setting it via SELECT INTO. That
seems like an enormous difference in speed, which I haven't really been
able to account for.

Test case:

andrew=# \d abc
Table "public.abc"
Column | Type | Modifiers
--------+---------+-----------
x | text |
y | text |
z | integer |
andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop
select 'a','b',i into r.x,r.y,r.z; end loop; end; $x$;
DO
Time: 63731.434 ms
andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop r
:= ('a','b',i); end loop; end; $x$;
DO
Time: 18744.151 ms

Is it simply because the SELECT is in effect three assignments, so it
takes nearly 3 times as long?

cheers

andrew