A bottleneck occurs when one task in a system limits the overall system performance to the extent that other tasks in the system cannot reach their performance potential. There is inevitably some task or tasks that limit system performance, but the goal is to balance the limitations to avoid over-engineering some tasks while other tasks become bottlenecks.
The motivation for adding hardware to accelerate OpenGL is to achieve better overall graphics performance. But if hardware is added in an unbalanced fashion, expensive hardware resources will go underutilized due to bottlenecks in the system and overall performance will suffer.