Why unsigned types are more efficient in arm cpu?

I'm reading an arm manual and come to this suggestion, but the reason is not mentioned.

Why unsigned types are faster?

Prior to ARMv4, ARM had no native support for loading halfwords and signed bytes. To load a signed byte you had to LDRB then sign extend the value (LSL it up then ASR it back down). This is painful so char is unsigned by default.

In ARMv4 instructions were added to handle halfwords and signed values. These new instructions had to be squeezed into the available instruction space. Limits on the space available meant that they could not be made as flexible as the original instructions, which are able to do various address computations when loading the value.

So you may find that LDRSB, for example, is unable to combine a fetch from memory with an address computation whereas LDRB could. This can cost cycles. Sometimes we can rework short-heavy code to operate on pairs of ints to avoid this.

There's more info on my site here: http://www.davespace.co.uk/arm/efficient-c-for-arm/memaccess.html

I think it's just that the instruction set for ARM CPUs is optimized for unsigned. Some operations can be done with one instruction for unsigned types but will need multiple instructions if it's signed. That's why I think if compiling for ARM in most (all?) C and C++ compilers it defaults to unsigned char rather than the more usual signed char.