Upgrading RAM with 2 different memory modules I have recently upgraded the RAM of my Macbook Pro mid 2012 which initially has 2GB &amp; 2GB in the slots to 2GB &amp; 8GB. Now, I have a 10GB RAM in it. After a day of upgrading, I noticed that the performance of the system has increased a bit. But will two different capacities in the slots bring a problem in the future?
DDR memory must be 'matched pairs' to get optimum performance.  That being said, any performance increase (which depends on the kinds of tasks you're running) from that pairing effect would be vastly outpaced by increasing your RAM from 4 to 10 GB, thus minimizing swaps to disk.  I would not worry about it especially on such an old machine.
More technical info here