The Microchip 93LC66B/P is a 4 Kbit serial EEPROM organized as 256 x 16 bits, accessed through a Microwire interface at clock rates up to 2 MHz.
2 MHz Microwire — what the clock speed means for your bus
The 2 MHz clock rate is the Microwire bus speed. Each read or write cycle shifts data serially at up to 2 MHz.
It is not suited for automotive under-hood, outdoor telecom cabinets, or unheated storage where temperatures drop below freezing. If your application sees -40°C or +85°C, look at the industrial-temperature sibling 93C66B-I/P, which shares the same 4 Kbit density and DIP footprint but extends the range.