That should be fast as long as the second table has an index on the timestamp column.
Personally, I think it's clearer without multiple CTEs:
<pre>
select id as id1,
(select id2 from
(select id2, delta from
(select t2.id as id2, t2.ts-t1.ts as delta from t2
where t2.ts>=t1.ts
order by t2.ts
limit 1)
union all select id2, delta from
(select t2.id as id2, t1.ts-t2.ts as delta from t2
where t2.ts<=t1.ts
order by t2.ts desc
limit 1)
order by delta
limit 1)) as id2
from t1;
</pre>