You might try enabling or disabling the "MacOS Shortcuts" option. (I think these instructions are still accurate: VMware Fusion 7 Documentation Center )
Leaving that on will sometimes cause us to delay key-events as we wait to make sure that you weren't entering a key-combo that we have to handle specially.
Although I think the "Fn" key specifically is handled by the hardware or OS before it makes it to us, so this might be a change in MacOS.
Thanks for the suggestion. I disabled "Mac OS Host Shortcuts", and found that it did remove the delay in processing keystrokes involving the 'Fn' key. Re-enabling the feature reintroduces the issue.
Host shortcuts are an integral feature allowing seamless transition between VM & Host OS. I don't consider disabling them a vaild fix; rather a troubleshooting step to help identify the cause of the delay. If anyone has any further ideas / steps which may help further identify the cause, please call them out, and Ill give it a try...