Are Bitcoin ASICs Optimized for Fixed Length Messages?
What do you mean by "really long"? If it's bigger than (2^64-1)/8 bytes (~2,097,152 terabytes) then it's not going to work anyway because this is the theoretical limit of SHA256 and on top of that it would take a huge amount of time to complete, but then again I am not aware of many computers that can hold such great amounts of data.  

For significantly smaller inputs it all comes down on the way the ASIC's SHA256 engine is implemented, what is the size of message (input) block in bits, how much memory is available per core in your SHA256 ASIC, etc. As per market standard, these cores are usually equipped with 128 kilobytes of memory. Of course nothing stops you from deploying external units of DRAM memory chips across memory-bus enabled ASICs but this will fire back in processing speed anyway.