slow _cat/aliases performance with large number of aliases + security

[inqueue]

Elasticsearch version: 5.6+master

Description of the problem including expected versus actual behavior:
_cat/aliases performs very slowly when cluster security is enabled. The issue was reported from a user upgrading 1.7.5 to 5.6.15 who noticed the API call was performing much slower on the later version. @imotov is able to reproduce on master.

Steps to reproduce:

1. enable security
2. create a 90000 aliases
3. call _cat/aliases

[elasticmachine]

Pinging @elastic/es-security

[jasontedor]

I am concerned about adding an "optimization" for this as 90000 aliases would generally be discouraged. There are many aspects of the system that would be slow with such a large number of aliases.

[jasontedor]

Also please note it that 5.6 is out of maintenance and would not see any additional updates at this point.

[tvernum]

We discussed this.
We do not intend to make a change to 5.6, nor do we really want to optimise for 90,000 aliases.

However, we thought that it might be possible to make it so that when run as superuser (or any other user with access to every index, including restricted indices), it mimics the performance that you get when security is disabled. That is, the performance impact comes from trying to apply security over the list of alias/index names, and there might be cases where we can optimize that.

However:

1. we don't expect a general solution that would apply all users
2. we don't know if the proposed optimisation is safe / possible
3. there are no immediate plans to prioritise this work