I apologize in advance by extreme simplification and excessive use of hyperlinks. Feel free to discuss whatever you consider relevant, using appropriate means.
Groovy = Java++
With time and maturity of the developer and the technology itself:
Groovy = Java **
Grails = (Spring Framework + Hibernate) **
You write the same kind of application, in significantly less time.
And if I discover that some piece of code in Groovy is affecting the performance of my application?
If Groovy is the real cause of the problem, use Groovy++ for that piece of the code;
If the result is not satisfactory, rewrite this piece in Java;
If the result is not satisfactory, rewrite it in C and inline-Assembly (and use JNI or JNA);
If the result is not satisfactory, build your own hardware or give up on programming.
I’m still not convinced, give me more information
Take a deep breath, read and decide for yourself:
And Google is your friend.