Large cubes have more internal friction. Using a high-quality silicone lubricant is essential to prevent "lock-ups" during long algorithms.
This teaches you how to handle "fixed" centers versus "moving" centers. Xnxnxnxn Cube Algorithms PDF Nxnxn Rubik Cube...
A 7x7 solve can take 5–10 minutes for an intermediate cuber; focus on piece look-ahead rather than raw turning speed. Large cubes have more internal friction