a) Faster clock (less time between steps) - measure in GHz or MHz
more difficult and more expensive to make faster cpu's.
b) Wider Bus (transfer more information to/from memory) - 16 bits/32 bits/64
more lines takes more space in the IC which could be used for
c) Cache Memory (try to save important information in faster memory)
use a small, very fast memory, very close to cpu
takes space, more expensive
hopefully you will find information on the cache most of the time.
d) Multiple CPU's
two or more CPU's (duo systems, quad systems)
not all problems can be done faster with more CPU's
e) different design