Users of Apple's iPhone can only compose text messages at half the speed achieved by users of conventional Qwerty and numeric phone keyboards, new research has revealed.

Chicago-based usability consultancy User Centric said that the iPhone's touchscreen is "potentially problematic" for sending text messages.

User Centric tested the iPhone's SMS features with frequent texters to see how rapidly they could adapt to the iPhone's touch keyboard.

All 20 participants sent at least 15 messages per week. Ten participants owned phones with a full Qwerty keypad and 10 owned phones with a numeric keyboard. None of the participants owned an iPhone.

Each participant typed six fixed-length text messages on their own phone and six on an iPhone.

It took Qwerty users almost twice as long to create the same message on the iPhone as it did on their Qwerty phone. While there was improvement over time, the difference persisted even after using the iPhone for 30 minutes.

"For Qwerty users, texting was fast and accurate. But when they switched to the iPhone, they were frustrated with the touch sensitive keyboard," said Jen Allen, a usability specialist at User Centric.

In contrast to Qwerty users, numeric users used the 'multi-tap' method of entering text messages on their phones, pressing individual number keys multiple times to get a desired letter or character to appear.

Although multi-tap is inherently inefficient, numeric phone users took nearly as long to create a message on the iPhone as they did on their numeric phones.

There was no increase in efficiency despite the iPhone's corrective text approach, User Centric found.

When using the iPhone's touch keyboard, all participants frequently selected keys that they had not intended.

Participants usually corrected these errors by using the backspace key to erase one character at a time. Only seven participants figured out how to use the corrective text feature on their own.