Severe performance degradation with jdk10 / java10

[Arno1]

as this is (presumably) a parallels issue, would you like to investigate further?

Since the VM settings (which are available in the problem report, that you've sent) are identified as the culprit and the reproduction scenario seems to be clear, I don't believe, that you need to waste any further time on this issue. I'll post an update though, if a particular setting yet to be identified turns out to be a designed behavior rather, than a defect.

ps. is there any way to get efi working again after swapping the hd image?

You mean getting rid of the system flag, that you have added, or booting Windows in the UEFI mode? The issue is that Windows installed in the legacy BIOS mode is not suitable for UEFI boot. I am not aware of any way to convert the existing installation. As a substitute to the system flag, VM EFI boot may be disabled by a CLI command:
prlctl set "Windows 10 Dev" --efi-boot off