If a lambda extension executes longer than the lambda execution, is it counted towards the duration of the lambda and against reserved concurrency?

For eg., if I have a lambda with 5 reserved concurrency and a lambda extension that runs twice as long as the lambda that takes X ms to execute, will the overall execution time be 2X ms? If I have 5 concurrent lambdas that start at the exact same time, will 2X ms (and not X ms) have to elapse before another lambda executes?

And would this apply to both internal and external extensions?