internationalizationkeyboard
If you hold down a letter then OS X will show you options for accented versions of that character. This is very useful when typing in other languages.
However, not all the versions of that letter are always available. For example, if I hold down the [S], then I can choose ß, ś, or š. However, I'd like for it to also display the turkish ş. Is there a way to add that character to the accent menu?
I'm looking for an easy way to type Turkish characters on my english keyboard.
I don't know a direct solution to your question: Reduced delay. If your iPhone is jailbroken you might be able to find the preference file that sets the delay and reduce it.
One work around could be to not deal with the accents at all and have the iPhone insert them for you, as autocorrected words. You can now type full speed.
To add custom words to the dictionary:
For example, I added Word=fooxx and Yomi=föoxx, I then went to the Notepad and using the English keyboard I typed "fooxx", and the iPhone suggested föoxx. When I did this example without the "xx" it did not work, perhaps because foo is already in my dictionary.
Alternatively, if you iPhone is jailbroken you could try using Xpandr
The characters included in the popovers are defined in property lists inside the /System/Library/Input\ Methods/PressAndHold.app/ bundle.
I was able to change à to y by editing /System/Library/Input\ Methods/PressAndHold.app/Contents/Resources/Keyboard-en.plist and logging out and back in.
(note the escape in "Input Method" if you're copy pasting)
<key>Roman-Accent-a</key>
<dict>
<key>Direction</key>
<string>right</string>
<key>Keycaps</key>
<string>a y á â ä æ ã å ā</string>
<key>Strings</key>
<string>a y á â ä æ ã å ā</string>
</dict>
Best Answer
You can find the answer where this question was asked back in 2011:
How to add characters to the press and hold character picker in OS X Lion?
An extra step is needed for El Capitan:
The keyboard's plists of PressAndHold also can no longer be modified to add the new accents in El Capitain