Overview
As you type at a bash prompt, the rest of the command you most likely mean appears dim after the cursor, and Right arrow takes it. Everything else you type just types.
The usual way to get this in bash, ble.sh, replaces readline with a line editor written in bash, which is why it is slow. inkling leaves readline alone: it runs the shell inside a pseudo-terminal it owns, asks readline itself what is in the line, and draws the suggestion next to what the shell draws. A key and its suggestion are on the screen as soon as bash's own echo of the key would be, however long the history.
What it suggests
The suggestion is the command you most likely mean here: from what you run in this directory, in this project and after the command you just ran, among those whose paths are there. With nothing in the history, a program's or a file's name is finished instead. Replayed over its owner's own history, the very command is suggested after one character 64% of the time, against 52% for fish's rule.
Case is no bother. A letter typed in lower case fits a command that has it in upper case, so cd ~/dev/gith finishes as cd ~/Dev/GitHub/, and taking the suggestion puts the case of the line right. A capital typed is kept. Typed all in lower case, the replay still has the very command after one character 63% of the time.