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Update: SPECK, considered in this other answer, is good if compactness or speed per encryption for narrow block size are the choice criteria. But I doubt that it is competitive with AES from the standpoint of cycles/byte on a Z80 (as asked in the question). In particular, SPECK's multi-bit rotation and large number of rounds are going to be show stoppers.

AES is near ideal for lowly 8-bit CPU because

Update: SPECK, considered in this other answer, is good if compactness or speed per encryption for narrow block size are the choice criteria. SPECK-32 can be competitive with AES from the standpoint of cycles/byte on a Z80 (as asked in the question) because the state fits in registers. But I doubt SPECK-128 can be competitive with AES from the standpoint of cycles/byte on a Z80, because the multi-bit rotation and large number of rounds are going to be show stoppers; and the state has to be in memory.

AES is good on 8-bit CPU (especially if a wide block is desired) because

Update: SPECK, considered in the other answer, is good if compactness or speed per encryption for narrow block size are the choice criteria. But I believe it is not competitive with AES in term for cycles/byte on a Z80 (as asked in the question); in particular, SPECK's rotation by 3 bits and large number of rounds are going to be show stoppers.

Update: SPECK, considered in this other answer, is good if compactness or speed per encryption for narrow block size are the choice criteria. But I doubt that it is competitive with AES from the standpoint of cycles/byte on a Z80 (as asked in the question). In particular, SPECK's multi-bit rotation and large number of rounds are going to be show stoppers.

Update: SPECK, considered in the other answer, is worth consideration if compactness or speed per encryption for narrow block size are the choice criteria. But I believe it is not competitive with AES in term for cycles/byte on a Z80 (as asked in the question); in particular, SPECK's rotation by 3 bits and large number of rounds are going to be show stoppers.

Update: SPECK, considered in the other answer, is good if compactness or speed per encryption for narrow block size are the choice criteria. But I believe it is not competitive with AES in term for cycles/byte on a Z80 (as asked in the question); in particular, SPECK's rotation by 3 bits and large number of rounds are going to be show stoppers.