have a collection

a,
b,
c

and index on a,b

found that when aggregation contains $or

$or:[{"a":xx},{"b":xx}]

is very slow,even both of a and b have index.

1. when query with "a",may need 300ms,
and the result is 3x larger than query with "b".
but when query aggregate with

$or:[{"a":xx},{"b":xx}]

it need 1.5s