Guest systems are slow when host system is creating new guest

Guest systems are slow when host system is creating new guest, and gain normal speed again after creation process accomplishes. I wonder is there anything I can do about this like limiting usage of CPU or RAM resources? Though despite some jumping in CPU usage it is generally ok. But RAM gets filled up, though buffer/cache has quite large room:

free -m total used free shared buffers cached Mem: 64398 64152 245 0 25 46363 -/+ buffers/cache: 17763 46635 Swap: 66591 515 66076

Am I experiencing RAM insufficiency? What buffer/cache sizes mean in this situation?