Improve performance reports

- Differences less than 10 seconds seem to happen quite regularly just due to random variations in GitHub Actions, so don't report performance changes if the difference is less than 10 seconds.
- Clarify that it's mypy that got faster/slower, not the package we're running mypy on :-)

mypy_primer.py

@@ -558,14 +558,14 @@ def format_concise(self) -> str:
         else:
             speed = "slower"
 
-        has_runtime_diff = runtime_diff > 5 and runtime_ratio > 1.05
+        has_runtime_diff = runtime_diff > 10 and runtime_ratio > 1.05
         if not self.diff and not has_runtime_diff:
             return ""
 
         ret = f"{self.project.name} ({self.project.location})"
         if has_runtime_diff:
             ret += (
-                f" got {runtime_ratio:.2f}x {speed} "
+                f": type-checking got {runtime_ratio:.2f}x {speed} "
                 f"({self.old_result.runtime:.1f}s -> {self.new_result.runtime:.1f}s)"
             )
         if self.diff: