Emulator timings----------------"Let's Emu!" is not a true emulator, it is more a Z80 opcode interpreter. It does not emulate any timings of the original machine, as this would require a stronger host machine: 65C816 running at 14 MHz is not powerful enough. So you cannot expect that timings will be even close to those you can observe on a real ZX Spectrum. This will have an ill effect on programs which rely on precise timings. Fortunately, most ZX Spectrum programs do not rely on timings and are less or more fine with the odd machine we can offer them.Now, to give you an idea on what you can expect: the fastest instruction is NOP, it takes 14 clock cycles on host machine, vs 4 t-states (clocks) on a true Z80 running at 3,5 MHz. How long this takes in wall clock time, this of course depends on how the 65C816 is clocked. General formula is:F = 65C816_clock / 3500000T = emulator_clocks / FExample: as said above, NOP takes 14 clocks. On a 14 MHz host machine, this gives F = 4, and T is 14 / 4, i.e. 3,5. This is the number of Z80 t-states (clocks) a NOP takes on our emulated machine. Since a true Z80 consumes 4 t-states to execute a NOP, we can see that at 14 MHz our machine will execute NOPs faster than on a real Spectrum. If the host machine is running at 7 MHz, the same NOP occupies time equal to 7 t-states of a real Spectrum, so it is much slower.Opcode decoding principles--------------------------Anyway, NOP is a special case. Any other (unprefixed) instruction takes 14 clocks instruction decoding plus some execution time, which is at least 3 clocks.A CB prefix adds another 14 clocks to instruction decoding.A DD, FD or ED prefix adds 15 clocks. If a number of identical prefixes is to be executed (e.g. DD, DD, DD, DD etc.), the first one adds 15 clocks, and every subsequent one - 14.A double prefix DDCB or FDCB is a more serious matter, as it needs to do address calculation. Therefore it takes 43-45 clocks to get decoded.Keep in mind that this is only the "decoding" time: to get an idea, how long an instruction will be executed, you have to add the actual execution time. For a NOP it is 0 clocks, but all other instructions have a nonzero number here, even if they virtually do nothing (like LD B,B and such).Execution time--------------Any non-prefixed instruction (except NOP, DI and EI) adds at least 3 clocks execution time.A CB-prefixed instruction also has at least 3 clocks execution time.Any other instruction has at least 4 clocks execution time.These considerations do not take two things into account:1) halts generated by ANTIC2) various synchronization issues between fast bus and chip bus which vary on different accelerators.Therefore timings given below should be considered minimal.Examples--------Instruction  Decoding  Execution  Total   Chip bus cycles  Comments-----------  --------  ---------  -----   ---------------  --------NOP          14        0          14      0DI           14        25         39      1EI           14        20         34      1LD A,A       14        3          17      0LD A,B       14        9          23      2CPL          14        16         30      3SCF          14        15         29      3RLCA         14        28         42      6LD SP,HL     14        11         25      4LD HL,$nnnn  14        36         50      2LD $nnnn,HL  14        30+12      56      2                for $5b00-$ffffLD $nnnn,HL  14        30+23      67      4                for $4000-$5affJR           14        34/33      48/47   4                forwards/backwardsJR Z taken   14        41/40      55/54   5                forwards/backwardsJR Z ignored 14        13         27      1CALL         14        46         60      4CALL Z taken 14        53         67      5CALL Z ign.  14        13         27      1JP (HL)      14        7          21      1JP $nnnn     14        19         33      0JP Z taken   14        26         40      1JP Z ignored 14        15         29      1RLC B        28        43         71      6JP (IX)      29        8          37      2RLC (IX+d),b 43/45     43+11      97/99   1                for $5b00-$ffffRLC (IX+d),b 43/45     43+18      104/106 2                for $4000-$5affAnd so on. I hope you get the idea.