The performance boost is not about ignoring security, but about ignoring edge case of one particular security check on i386 which ended up being the only critic

The performance boost is not about ignoring security, but about ignoring edge case of one particular security check on i386 which ended up being the only critical one for "modern" unix-derived (which for purposes of this discussion includes Windows NT) OSes. In original x86 protected mode model (ie. real security flags on descriptor level and page protection bits only used for scheduling of paging) this would be non-issue.