From SpecNext official Wiki
Acceleration, or Turbo mode, allows the Next to run at faster clock speeds than a standard Spectrum.
- Turbo off, or turbo on and $07 set to #00: 3.5 Mhz (standard clock).
- Turbo on, $07 set to #01: 7 Mhz (x2). (Standard for Pentagon).
- Turbo on, $07 set to #10: 14 Mhz. (x4) Some features, such as Layer 2 (vertically visible pixel area) and shadow-VRAM of ZX128 (whole pixel area), will automatically throttle down to 7MHz while specified areas of display are sent to the video signal.
- Turbo on, $07 set to #11: 28 Mhz. (x8) - this mode is removed for the moment, due to HW limitations (the system was not stable enough at 28MHz).