Query slows with additional FROM item (even with no extra actions)
As far as I understand if you want to know which is % of 'deleted' rows, do not cross join with the same table, get total rows and calculate the result on your initial query.

with totalc2 as 
(
    select count(*) nrows
    from comment
)
SELECT CAST(COUNT(c1.body) AS float) /totalc2.nrows) as percent_deleted
FROM comment c1
WHERE c1.body LIKE '%[deleted]%' 


You can use it as a subquery:

SELECT CAST(COUNT(c1.body) AS float) / (select count(*) from comment) as percent_deleted
FROM comment c1
WHERE c1.body LIKE '%[deleted]%'