Speedlock is a tape and disc copy protection solution.
8K Speedlock refers to a specific version of the Speedlock where most of the data was stored on special tracks.
Each track was setup with NEC765 sector size 6, which indicated a sector with 8192 (&2000) bytes of data. However in reality, only 6146 (&1800) bytes were useable. Recreating this format on the Amstrad was difficult because the NEC765 would really try to write 8192 bytes of data, and in doing so it would corrupt the MFM data structure used to layout the sectors on the track.