Attempting to remove 3 bytes from the block header would likely render all ASIC mining hardware worthless, at a saving of 200kB per year of disk space.
- I am not an expert, but I googled and couldn't find evidence.
(compression seems very underutilized)
Most of the contents of the block chain are hashes (uncompressible) and ECDSA signatures (uncompressible).
- How come? The header clearly not.
- Also, from what I read the transactions aren't either.
Thus, most of the volume is uncompressed.
However, you are right, compression would require more CPU cycles, but simple reductions could come at a very low cost of maybe 0.1% performance. Nothing in comparison to the saved bandwidth and HDD space.