I’m using virtualbox on Ubuntu as a host and debian 10.4 as a guest. My connection is awful for the guest. I’ve learned it is because of…

Note: Bridging to a wireless interface is done differently from bridging to a wired interface, because most wireless adapters do not support promiscuous mode. All traffic has to use the MAC address of the host’s wireless adapter, and therefore Oracle VM VirtualBox needs to replace the source MAC address in the Ethernet header of an outgoing packet to make sure the reply will be sent to the host interface. When Oracle VM VirtualBox sees an incoming packet with a destination IP address that belongs to one of the virtual machine adapters it replaces the destination MAC address in the Ethernet header with the VM adapter’s MAC address and passes it on. Oracle VM VirtualBox examines ARP and DHCP packets in order to learn the IP addresses of virtual machines.

https://download.virtualbox.org/virtualbox/6.0.20/UserManual.pdf Page 116

And then…

Linux hosts. Functionality is limited when using wireless interfaces for bridged networking. Currently, Oracle VM VirtualBox supports only IPv4 and IPv6 over wireless. For other
protocols, such as IPX, you must choose a wired interface.
Also, setting the MTU to less than 1500 bytes on wired interfaces provided by the sky2
driver on the Marvell Yukon II EC Ultra Ethernet NIC is known to cause packet losses
under certain conditions.
Some adapters strip VLAN tags in hardware. This does not allow you to use VLAN trunking
between VM and the external network with pre-2.6.27 Linux kernels, or with host operating
systems other than Linux.