I would recommend to move this to your query as it will be easier and faster. Also, adding an expression on a cell in the repeating section will always make it slow.
You can also try
=IIF(MAX(Fields!CPU.Value) > MAX(Fields!RAM.Value), MAX(Fields!CPU.Value), MAX(Fields!CPU.Value))
In general if you don't have to do anything with data in other rows then a simple if statement should work.
=IIF(Fields!CPU.Value > Fields!RAM.Value, Fields!CPU.Value, Fields!CPU.Value)