slow live migration ram

Hello, I am using openstack rdo with kvm libvirt. Live migration is very slow when the vm is using ram intensively. If I migrate a vm that has been running on a host for a few days, it takes several minutes to migrate the memory. After the first migration, I immediately try another one on the same vm, it takes very little. I think it's due to the fragmentation of the pages: is there any remedy? Thanks Ignazio