Direct sql queries can be used to improve performance of your autocomplete when working with big amount of data.
We can use {{input}} token in a query, it is replaced by input, so all work will be done on a sql server side.
To do this, follow steps described in SQL Datasource and then use one of a listed queries or make your own.
Project Server 2013, autocomplete from a list of users in Team Members security group:
SELECT
resources.RES_NAME
FROM pub.MSP_WEB_SECURITY_GROUP_MEMBERS members (nolock)
join pub.MSP_WEB_SECURITY_GROUPS groups (nolock) on groups.WSEC_GRP_GUID = members.WSEC_GRP_GUID and WSEC_GRP_NAME = 'Team Members'
join pub.MSP_WEB_SECURITY_CLAIMS secguids (nolock) on secguids.SECURITY_GUID = members.WRES_GUID
join pub.MSP_RESOURCES resources (nolock) on resources.WRES_CLAIMS_ACCOUNT = secguids.ENCODED_CLAIM and resources.RES_NAME like '%{{input}}%'