It's not even close to worth it, as you will never notice the difference.
CPU upgrades on older systems are probably the worst upgrade you can make, as the platform itself will still be old and slow. Even newer platforms are still a bad fit for CPU upgrades, even major ones.
I upgraded a Latitude E6540 from a dual-core i5-4200M (probably the slowest CPU offered with this model) to a quad core i7-4810MQ, and other than more heat and fan noise, I really don't notice the difference in 99% of tasks. There are simply too many other variables at work to make a CPU upgrade on an older system remotely worth it and this was a MAJOR upgrade as for as CPUs go.
A CPU upgrade makes sense in our minds, but the reality is a far different story. You need to accept that this is the best your old system is going to perform, and take any potential upgrade money and save for a new PC or laptop. Tossing money at a system this old is just a waste of cash.
I recently bought Optiplex units with i5-8500 and i7-8700 (6 core with HT) processors and those puppies just fly. It burns through any job I toss at it, and buying current-gen hexa- or octa-core models represents a real upgrade, as everything about the system is faster. Heck, I even game a bit on the onboard GPU.