Profile your existing app and find out what is making it slow.

When you fully understand how your existing app performs (and why it performs) you may start making informed decisions.

Profiling is a highly technical art that is beyond even most competent programmers. You need a good systems engineer who is proficient with the tools used to examine application performance on your system, and understands the different technology stacks involved (networking, disk I/O, cpu/threading/scheduling, virtual memory etc.). If you can reproduce the problem, they should be able to pinpoint the cause.

Also, you may want to read up on Node.js and understand what benefits it brings to the table - it is not a silver bullet that solves all Web application problems.