Jody has an old iMac with a 2.66GHz dual core processor. Leo says that's still a useful computer, even with its age. Jody says Lightroom stalls on it. Leo says that's more the fault of Lightroom. But a new iMac would also give him a larger screen and far more accurate color depiction. A new iMac will still have its hands full with Lightroom because its just a pig with memory resources. It's poorly written for today's modern platforms. Here's what to try before Jody buys a new iMac: Install a Solid State Drive (SSD). That will speed up the performance dramatically. Then use the optical drive as a data drive. He should also update the RAM with as much as he can afford. That won't solve the shortcomings of the old graphics processor, but it's a start.
Buying a new iMac Pro would also help, if he's ready to upgrade. But even with that, Lightroom can bog down.