Why are apt downloads so slow in my VM?

First compare the VM with the host using a speed test or a large download. If the host is also slow, the VM probably isn't the cause. If only apt is slow, try a different mirror and check whether the problem changes over time. A mirror can be slow or busy even when your general connection is fine.

Check VirtualBox's network mode and any bandwidth limits in the VM settings. Bridged and NAT can both work, so switching modes isn't a guaranteed fix, but checking for an accidental cap or unusual network configuration is worthwhile. Also confirm the guest can reach other sites at normal speeds.

Since you've already tried nearby mirrors and IPv6, I'd avoid repeating those changes for now. Note the actual download rate and whether it affects all packages or only some. If it comes and goes, a mirror or route issue is plausible. Slow speeds reported on other Debian-based systems, including physical machines, suggest it isn't necessarily a VirtualBox-specific problem.

Check the configured package sources and make sure they point to a current, reachable mirror for your distribution and release. Changing a mirror can help, but it won't fix a limit elsewhere on the network. Compare apt downloads at different times and test a download outside apt before narrowing it down.