FULL PRODUCT VERSION :
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [version 10.0.16299.192]

A DESCRIPTION OF THE PROBLEM :
This is an upstream bugreport for https://josm.openstreetmap.de/ticket/15782. As per Quality Outreach please add josm-found label to this bug report.
We noticed that JOSM takes a lot longer to start with Java 9/10 and WebStart.

With Java 8:
.\jre1.8.0_162\bin\javaws.exe https://josm.openstreetmap.de/download/josm-latest.jnlp
=> JOSM starts as expected.

With Java 9:
.\jre-9.0.4\bin\javaws.exe https://josm.openstreetmap.de/download/josm-latest-trace.jnlp
=> JOSM is a lot slower to start.

In https://josm.openstreetmap.de/ticket/15782#comment:6 I narrowed the performance regression to com.sun.deploy.net.protocol.jar.DeployURLConnection.getInputStream()

But as Java Web Start is not open source, I cannot investigate further.

REGRESSION. Last worked in version 8u162

ADDITIONAL REGRESSION INFORMATION:
java version "1.8.0_162"
Java(TM) SE Runtime Environment (build 1.8.0_162-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
With Java 9 or 10:
javaws.exe https://josm.openstreetmap.de/download/josm-latest-trace.jnlp

Notice how long it takes to start, unlike Java 8.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Same performance than Java 8
ACTUAL -
Startup time longer with Java 9

REPRODUCIBILITY :
This bug can be reproduced always.