mobility is the average speed of electron in semiconductor,so when avg speed of electron slows so speed of device goes down
i think so
but holes has very less mobility compared electrons but when there is vacancy there will be sudden spurt in electrons to fill the gap so while hole mobility relatively less(not too much)

yes. that's why n-type devices are having faster operating performances comparitively than p-type devices.coz holes having lesser mobility than electrons due to weight.and now even more faster operating speeds are acheiving with electrons than before by varying the spin of the electrons.
this is called SPINTRONICS and is still under R&D. the size of the wafer also can be reduced.please refer to search engines for more information.