Photoshop, being a modern 64-bit applications is able to take advantage of your machine's computing power as most as possible.

However, did you know that there were a setting limiting the amount of RAM the software uses? In fact, this doesn't allow Photoshop to use more than 70% RAM by default.

You can change this setting by going into the Preferences panel and to Performance there. I usually set it to use up to 90% of my RAM in order to let 10% for the OS and other programs. If you don't have a ton of memory, you should lower the amount of RAM the program can use.

Hopefully, this will speed up Photoshop processes such as filters and input/output of files and also lose the laginess of the program when there is a lot of layers in your project.