Slow live snapshot during high guest loads?

[Notorand.IT]

[UPDATE below!]
If I take I live snapshot of a VM during its mid-to-high load (loadavg is about 3.5 on a 4-cpu VM) it takes ages before finishing. I am talking about 4 to 6+ hours.
The guest has 12 threads and 4 times the VM RAM. The VM is the only one running.

On a twin guest/host setup but with a VM having a low load (loadavg around 0.1) it takes about 30 minutes.

Is this normal? Why is it behaving like that?

TALIA!

[UPDATE]
It looks the long delay is not closely related to the guest workload.
On twin hosts taking the snapshots of similar guests (same amount of RAM/CPU/storage, more or less same workloads) take way different timing for snapshots.
One takes 25 to 30 minutes, one takes from 4 to 8+ hours.
What should I check on hosts/guests to try find a fix/workaround?