EXPLAINでコストを見てみたらIS NULLのコストはIS NOT NULLより高いことがわかる。
EXPLAIN
SELECT * FROM public.note
WHERE "userHost" IS NULL
ORDER BY id DESC LIMIT 100
--Limit (cost=0.56..6554.39 rows=100 width=1126)
EXPLAIN
SELECT * FROM public.note
WHERE "userHost" IS NOT NULL
ORDER BY id DESC LIMIT 100
--Limit (cost=0.56..22.92 rows=100 width=1126)