Steps to reproduce:

(Preconditions - Windows 7 with latest patches installed and running; Oracle's Open JDK 11 downloaded and on local file system; Oracle's JDK 1.8.0_191 downloaded and on local file system)

1. In SpringToolSuite4.ini, enter line -vm <path-to-Oracle-OpenJDK11.0.1> or set as Windows' default JDK via PATH and JAVA_HOME environment variables.
2. Execute SpringToolSuite4.exe
3. Add another JRE to STS
   a. Click Window->Preferences
   b. Click Java->Installed JREs
   c. Click Add...
   d. Click Standard VM
   e. Click Next>
   f. Click Directory and navigate to bin directory of Oracle's JDK 1.8.0_191
   g. Click Finish
   h. Click Apply and Close
4. Create a Java project
   a. Click File->New->JavaProject
   b. Click "Use a project specific JRE"
   c. Select the jdk1.8.0_191 added in step 3.
   c. Click Finish
5. Create a Class
   a. Right click on project
   b. Click New->Class
   c. Enter a class name
   d. Click Finish
6. In the editor, type in the full package name of a class.
7. Use backspace and change to a different.
8. Repeat step 7 several times.

At step 7, system starts to lag and eventually freezes. Note: Issue occurs more rapidly if Java 11 Support for Eclipse 2018-09 is installed. Consistently reproduced on Windows 7. Unable to reproduce on a Mac High Sierra.