Compiling a generated file with about 50 thousand imports compiles in under 10 seconds with JDK 8, and 3-4 minutes with JDK 9. The example below is based on the output of a real-world code generator.

A program that generates the test inputs is attached. It creates a large number of classes in unique packages, and two source files:
A.java imports all of the generated classes and creates variables for each of the imports.
B.java creates an equivalent number of locals but doesn't import anything.

Example:

$ javac Gen.java && java Gen 2

=== ./A.java
import p0.Tp0;
import p0.p0.Tp0_p0;
import p0.p1.Tp0_p1;
import p1.Tp1;
import p1.p0.Tp1_p0;
import p1.p1.Tp1_p1;

class A {
  void f0() {
    Tp0_p0 Tp0_p0_;
    Tp0_p1 Tp0_p1_;
    Tp0 Tp0_;
    Tp1_p0 Tp1_p0_;
    Tp1_p1 Tp1_p1_;
    Tp1 Tp1_;
  }
}
=== ./B.java
class B {
  void f0() {
    Object Tp0_p0_;
    Object Tp0_p1_;
    Object Tp0_;
    Object Tp1_p0_;
    Object Tp1_p1_;
    Object Tp1_;
  }
}
=== ./p0/p0/Tp0_p0.java
package p0.p0;
public class Tp0_p0 {}
=== ./p0/p1/Tp0_p1.java
package p0.p1;
public class Tp0_p1 {}
=== ./p0/Tp0.java
package p0;
public class Tp0 {}
=== ./p1/p0/Tp1_p0.java
package p1.p0;
public class Tp1_p0 {}
=== ./p1/p1/Tp1_p1.java
package p1.p1;
public class Tp1_p1 {}
=== ./p1/Tp1.java
package p1;
public class Tp1 {}
===

# Repro:

# generate the inputs
$ javac Gen.java && java Gen 6

# pre-compile the files to import
$ find p* -type f | xargs javac

# compile A.java and B.java with JDK 8 and JDK 9 javacs

$ time javac -fullversion -sourcepath : -cp . B.java
javac full version "1.8.0_162-ea-b03"
real 0m1.532s
user 0m3.708s
sys 0m0.692s

$ time javac -fullversion -sourcepath : -cp . A.java
javac full version "1.8.0_162-ea-b03"
real 0m6.271s
user 0m20.800s
sys 0m3.424s

$ time javac -fullversion -sourcepath : -cp . B.java
javac full version "9.0.1+11"
real 0m11.201s
user 0m9.236s
sys 0m11.992s

$ time javac -fullversion -sourcepath : -cp . A.java
javac full version "9.0.1+11"
real 3m56.710s
user 2m49.572s
sys 1m32.724s