I'm presuming it's just bios update to disable certain optimizations within the CPU.. meaning they were already in place to be A/B tested.. just flipping bit on/off.
Sometimes the microcode can do simple patches like inserting a nop between problematic instruction combinations. This usually doesn't have much (if any) negative performance implications. The instruction decoder (again AFAIK) is fairly programmable.
However if there is a more serious flaw in the actual silicon the microcode must rewrite the problem instructions to emulate the correct behavior and that can make it extremely slow - think emulating floating point but not quite as bad.
// Minimum Microcode file.
cond OP:4;
cond uaddr:3;
signal /MEMIO = ....1;
signal MREAD = ...1.;
signal MWRITE = ..1..;
field REGMOVE = XX___;
signal MAR <- PC = 01...;
signal IR <- MEM = 10...;
start OP=XXXX; // A tiny fetch instruction
/MEMIO, MREAD, MAR <- PC; //
hold; // Same as MEMIO, MREAD, MAR <- PC
hold, -MREAD, IR <- MEM; // Same as MEMIO, MAR <- PC, IR <- MEM
// End of file.