You'll never achieve a better per-process performance - you can't double the clock speed of a single Raspberry Pi in that way.
Depending on the software that you want to run, you can create a cluster of Pi's which communicate with each other (probably via the Ethernet interface) to run certain tasks, and these tasks can then be run in parallel on every node in the cluster. In other words: you can achieve horizontal scalability (more nodes in a cluster), not vertical scalability (a single node can never be made faster).
Google around for 'Docker Swarm on a Raspberry Pi Cluster' for example, there are numerous examples and projects out there. Docker is a piece of software that allows you to run containers (small 'virtualized' applications), any piece of software that is containerized in a Docker image (see www.docker.com and this article) can then be run either on a single Pi, or in a redundant fashion on a cluster of Pi's.
Note: I completely agree with @drewrip's answer: Raspberry Pi's are not the ideal platform for this - they have some limitations which you must be aware of. You can read about some of the drawbacks (w.r.t. the OS, the flash drive, power management etc.) here.