Turning on --enable-preview (Loom) causes a set of performance regressions due to different inline tree.

Some microbenchmarks:
org.openjdk.bench.java.lang.StringBuilders.fromLatin1String
regression from 9 ns/op - 18 ns/op

org.openjdk.bench.java.lang.StringBuilders.fromLatin1StringBuilder
regression from 15 ns/op -> 25 ns/op

org.openjdk.bench.java.lang.reflect.Clazz.getMethodDifferentClassLoader
regression from ~240 ns/op -> 280 ns/op
and many others.

The reason for the performance regression is the fact that C2 generates post call noop instructions (required for Loom) which increases the instruction size of generated methods and some methods became higher than InlineSmallCode threshold.

Some typical non-inlined hot methods:
java.lang.AbstractStringBuilder::<init>
Class::getMethod
etc.