How to speed up old MacBook Pro15 inch late 2007 with 4GB RAM (2.4 GHz)

I am running 10.9.5 (Mavericks) and have about 12GB available on my hard drive with 2.2/2.4 GHz.

I have noted that performance has been getting slow (esp. on startup) and was wondering if there are any obvious ways to improve performance?

I would greatly appreciate your advice! Perhaps this question has been asked before but I could not find it using the search function.

Best,

Hirek