Do you have a long enough backup window to run a periodic active full? It sounds like, if you were doing synthetic fulls before, that you have enough available storage.
A large VM with that high of a change rate is going to have issues with any kind of synthetic operation. By doing periodic active fulls you can remove the transform operations which will cut your processing time considerably.
If you need to keep running synthetic operations then you will probably need to look into higher performance storage for your backup repository (fast disks in RAID-10 for example). You may also need to beef up your repository hardware.
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