We should not also forget about the printed circuit board layout. The lines are liable not only to external noise, but also to cross-connection when the SDA signal is induced in the SCL line, and vice versa. If the total length of the line is bigger than 10 cm, it is recommended to have the conducting paths in the following sequence:
It’s good to use printed circuit boards with separate layers, specially allocated for a return wire and Vdd wire, which is hardly feasible in amateur conditions. It’s also felicitous to use shielded cables, the screen conductor of which is connected to the return wire of the circuit. It is also possible to use twisted pairs of conductors.
As it was mentioned earlier, the transmission rate of the I2C bus is limited not only by external factors, but also by the internal interface parameters, their circuit technology. A developer of basic circuits is hardly capable of correcting the interface to make it work faster. This task can only be solved by a developer of integrated circuits. But it will be useful even for an amateur to find out why this restriction is imposed.