The CPU cycles are a non-issue for the configuration on this mainframe, and it's fair to assume that, since IBM developed the silicon and IBM engineers wrote the hypervisor layer, that most of the detrimental effects of "we need this to work across multiple CPU vendors and multiple generations of the architecture" are covered.
Even x86 CPUs get improvements to cache coherency in memory dedupe scenarios with generational improvement, and intelligent NUMA topology layout helps a lot. It's also worth noting that it's a mainframe, so essentially every configuration has a large number of other processors with a couple of instructions disabled so they don't "count" as CPUs for licensing, but they still perform work. I would not be surprised in the least if there were 1+ co-processors dedicated to offloading operations exactly like "LPAR || z/VM memory dedupe"
http://www.redbooks.ibm.com/redpapers/pdfs/redp4827.pdf
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It is called active memory sharing in the POWER world
https://www.ibm.com/docs/en/power9/9080-M9S?topic=sharing-ma...
0: https://www.kernel.org/doc/html/latest/admin-guide/mm/ksm.ht...