An integrated circuit 2 includes a transistor 26 which has a normal switching speed arising during normal operations of that transistor that apply electrical signals within normal ranges. If it is desired to change the speed of operation of the transistor, then speed tuning circuitry 12 applies a tuning electrical signal with a tuning characteristic outside of the normal range of characteristics to the transistor concerned. The tuning electrical signal induces a change in at least one of the physical properties of that transistor such that when it resumes its modified normal operations the switching speed of that transistor will have changed. The tuning electrical signal may be a voltage (or current) outside of the normal range of voltages applied to the gate of a transistor so as to induce a permanent increase in the threshold of that transistor and so slow its speed of switching. Temperature of a transistor may also be controlled to induce a permanent change in performance/speed.