Watch out for long I2C wires to your slave.
Also if you are using Arduino IDE, I have a strong feeling that the library when used slowly, it is all ok. But when fast, there seems to be spike generated. I am sure it is software because the spike is not random and always happens at the same place.
Get a scope and see, logic analyser that is sampling the I2C signal might not be able to capture these spikes. Also the spike could be legal (end of a I2C cycle) or real noise, without seeing it, its a bit hard to diagnose.