Is there any more way to squeeze out some performance out of my mid 2009 Macbook Pro 13"?
I found out that there is no more way to upgrade in terms of hardware since the last option would be a new CPU which cannot be replaced in the Macbook. Overclocking the CPU is no option either for safety reasons.

However, upgrading Mac OS from Yosemite to El Capitan has greatly increased performance, so I guess I'm good for now.