"Is key" or "is the key"?

"Is the key" implies that it is the only, or most important, factor. "Is key" means it's important. Which did you mean?

When something "is key" it is essential ~ if "concurrency" is essential, then you used the word properly...

if concurrency is only preferable but optional then "is key" doesn't make sense...

Regarding "is the key"

that wording would be correct if you said e.g: "concurrency is the key to making all rivers flow at the same speed" (yes, i'm playing with currents) but you could also say: "when making all rivers flow at the same speed, concurrency is key" ...it really depends on where in the sentence "key" appears. hope that's clear.