How can I tell when a faster memory card no longer improves my camera’s write performance?

A practical way is to measure the camera’s actual sustained write speed yourself.

Set the camera to its largest RAW file size, then shoot a burst of 5–10 frames to fill the buffer. Time how long the card-write light stays on while the camera finishes writing. Then transfer the files to a computer and note their total size.

Approximate write speed = total file size ÷ write time

That gives you the real write throughput of the camera-plus-card combination. If a faster card doesn’t increase that measured speed, you’ve likely reached the camera’s internal limit.

Also note that advertised card speeds can be misleading: a “95 MB/s” rating is often the read speed, while write speed may be lower. So the card itself may already be slower than its headline number suggests.

If you mostly shoot single JPEGs with pauses between shots, card speed usually matters very little; it matters most for long RAW bursts and buffer clearing.