I do not think its the problem with OS as wM 9.8 will work like a charm on Windows 8.x versions I have it installed for many users and they do not have any issues.
You may try to re-install the webMethods and select the components that you need for your working. I see you have selected all the components that comes as part of the free trial. Installing all the components may consume memory and makes your designer slow.
Let me know if you still have issues.
#webMethods-General#Integration-Server-and-ESB#webMethods