Comment 35 for bug 947664

I think you are on to something here.
I will try this solution on next kernel update.

I also found some information why this setting is not turned on by default:
https://www.electricmonk.nl/log/2016/03/14/terrible-virtualbox-disk-performance/

I have also created an issue at the VirtualBox forum (Kernel updates in VM with Ubuntu 18.04 host (kernel 5.0.0.x) takes very very long time):
https://forums.virtualbox.org/viewtopic.php?f=7&t=94822&sid=86f403dbb1ae5474d61565f11d33486c

/My regards from Sweden
Roger