I have three MBPs, that have the following OSXs installed: Mt. Lion, Mavericks and El Capitan. They all run efficiently and I cannot discern any meaningful differences among them in terms of response times.
If you have a concern about support, then I advise you stay with El Capitan and just make certain that your MBP is configured to maximize performance. As I indicated before, by posting an Etrecheck report is a good place to start. Also describe any specific symptoms that you are experiencing and any other relevant information:
http://www.etresoft.com/etrecheck
Even though though Snow Leopard is 'old', there still are many people still using the OSX successfully.
Ciao.