One vst synth = half the cpu power ?


I have a weird issue.
Obviously, I’ve checked with the Task manager and the resource manager, there’s no heavy cpu load, plenty of memory.
windows and drivers are updated, SMART indicating all harddrives healthy.
Running cubase, recording, all good. Yet, playing one chord on Sylenth1, Padshop2 or Vacuum, Massive X, Retrolog, or any vsti, and it jumps over 50% !
Few seconds of play time and it’s all lagging like crazy.

Now, I have a fairly good PC, it should handle a couple of vsts, doesn’t it?
Any idea what is wrong ?

What’s the asio buffer set to and what speed was the CPU at when you checked?

CPU IS 3700, always.
block/buffer on 1024