Adelina Țucă writing for Smashing Magazine:
Do not underestimate the impact of image optimization. Images are always one of the main reasons for a slow website. Google doesn’t like slow websites and nor do your visitors and clients.
This is the type of stuff that’s pretty awesome with WordPress. Most issues you could be having, there’s most likely someone already out there with a solution. I went ahead and installed Adelina’s suggestion of Optimole and saw immediate performance gains. If you use WordPress and love to blog with photos like I do, give this article a read.