SQL Server Linked Table in Access DB
Yes, Access on the client machine will connect directly to the linked tables located on SQL Server (or whatever ODBC back-end you're using). If you're dealing with forms and reports bound directly to these linked tables, it shouldn't cause any major performance problems. If you've written more complex queries within Access, then performance can depend on how well those have been optimized.

We've got a number of small Access "apps" running against SQL Server. They're great for when you just need to bang out a simple CRUD tool for a few people in an hour or two. Haven't had any real performance problems with them, though occasionally we run into a lock being held longer than it should.

My advice would be to take all the tables stored in that Access back-end and migrate them to SQL Server as well. Shared Access data files tend to be awful for performance and reliability.