Presumably “V4.3” is a reference to v4.0.3.3, which is the current version as I write this. You also do not mention the file format of the files i.e., ODS or XLS/X? In this case though I do not think this is likely to be a factor.
Initially I could not find any related open bugs (either for multiple file openings or Calc CPU usage). But then I found fdo#54350 which does indeed appear to describe your situation. Unfortunately the original reporter appears to have discovered that it may not be an LO problem but rather an operating system issue. It could very easily be a Heisenbug (i.e., difficult to replicate) or not an LO problem. Any information you could add to that bug (that is not already listed) would help.
If I had to guess I would say that either: a) the LO install may not have worked properly (in which case try un-installing, cleaning your registry, and re-installing); b) your User Profile is corrupt (in which case you can try resetting the profile); c) your Win7 install has a more serious problem.