In light of https://www.drupal.org/node/2470679#comment-10001385 you are literally saying "D7 is fast, your custom modules and code need not to be so"
Fast to run or fast to code? OOP makes complex systems easier to maintain, more robust and more sane to understand. In other words, it makes big, complex and mission critical systems cheaper to develop and maintain and with a higher level of quality in every sense of what quality can mean.
Slow down with the OOP kool aid, in PHP userspace it has a performance drawback.
I don't want to raise the argument into how badly broken (and more stuff that just feels bad to say) is PHP itself or the PHP ecosystem. You are very right in pointing out that PHP - more specifically the Zend Engine - is the hell of an unperformant piece of software. OOP was invented to save development time (and many other things). Other languages were designed from the ground with OOP in mind, not patched to support it as it happened with PHP.
This, of course, is much less pontificated than the "spaghettiness". Also, using functions instead of methods has *nothing* to do with spaghettiness, cyclomatic complexity has. https://github.com/symfony/DependencyInjection/blob/master/Loader/YamlFi... how is this not spaghetti?
I probably missused the term spaghetti: