We recently upgraded our production servers from 3.2 to 3.6, and started noticing huge CPU spikes and long transactions (60s+) on code that used to not cause issues.

Backing collection has about 2000 documents.  The (indexed) description field is a block of up to 512 characters at most.  The normal query has some other filters that narrow the matching set down to about 800 documents, and then this clause is the critical feature (driven by incremental search from web clients):

{{
{
  "description" : /(?=.*first)(?=.*second)/
 }
}}

We add more terms as the user types them.

In our old 3.2 mongo, this query took about 30ms at most.  On 3.6, just two terms takes over 30000ms, and three terms start to be over 70000ms. It's worst when the first term(s) actually match!

Rewriting the query to

{{{
  "$and" : [
    {
       "description" : /(first)/
    },
    {
       "description" : /(second)/
    }
  ]
 }
}}

was slower on 3.2 (from 30ms for the combined regex to 45ms for the $and clauses) but much, much faster on 3.6 (from 30000ms to 70ms).