Is DES slow in hardware or only in software? I have been reading that DES is slow.  Is it only slow in software or slow overall?  Also, as far as Triple DES being slow is concerned, is double AES with two 128 bit keys slower than single AES with a 256 bit key?
DES is slow in software because it was designed back in the early 70's even before the 8086 processor existed, and uses several bit oriented operations that are just not implemented efficiently in a processor with a word oriented instruction set.

Its intended product was ASIC hardware designs, in which DES runs quickly. DES hardware processors are quite common on older network cards intended for use in servers and workstations (I still have LAN card with wire speed TDES hardware). DES was already considered obsolete by the time the technology would have trickled down into consumer grade hardware.

DES becomes even slower when we compare it from a security standpoint against AES. DES in software is probably half the speed of AES, and it is also half the key space. Therefore TDES is used to increase the key space, but now it is 1/6th the speed of AES. AES is also better designed to resist attacks other than brute force.

Double AES with two 128 bit keys requires about 1.5X the processing of AES with a 256-bit key, and at least from a brute force perspective is not as secure. The reason TDES is used instead of double DES is because it requires 3 operations in order to double the key space.