Q:

Using /MD library with /MDd application

I have a library i need to use in release mode since it's performance is reduced significantly without optimizations. 
When i use it with /MD in my /MDd it gives crt warning, which i figure it is bad. ( may cause heap corruption?) 
so what i did is, keep all the release optimizations but link library with /MDd. 
warning disappeared.  
however, could this cause any problems or performance loss? 

A:

It can cause performance loss to use /MDd instead of /MD, but this definitively depends on what you do.  If you don't use the runtime in your performance-critical code anyway, you will not see any slow down. 
Alternatively you could solve your problem the other way around: Build application and library with /MD, but the application still with debug information and without optimization.