How fast was the ENIAC?

Answer 1

Each Accumulator could do 5000 additions or subtractions per second and there were 20 Accumulators that could potentially all run simultaneously if the problem could be setup with enough parallelism (which was difficult to do and thus rare, even when it could be done it was impossible to sustain for long).

The theoretical ideal case speed (with full parallelism) was 5000 * 20 = 100,000 additions or subtractions per second.

Answer 2

It could do 5000 additions/subtractions per second in each accumulator. It had 20 accumulators and could run several at the same time.