I have a .Net solution with 5 projects in c# and one in visual basic. I want to know whether there would be a gain of performance to convert my visual basic project to C# in the day to day while I compile my solution.
No. Visual Basic and C# projects both compile very quickly - so there is likely to be little difference. Also, since they both compile to the same or similar IL, there will be little difference in runtime performance.
However, there may be an advantage in terms of long term maintainability. Having a single language code base is often easier to maintain over time, especially with multiple developers.