The ZX Spectrum had (primitive) video hardware. The GP commenter means the ZX80 and ZX81 which used the Z80 CPU to generate the display and so really were unable to both "think" and generate the display at the same time. On the ZX81 there were two modes, SLOW mode and FAST mode. In FAST mode the Z80 CPU prioritized computations over generating the display, so the display would go fuzzy grey while programs were running, then would reappear when the program ended or it was waiting for keyboard input.