The 555 timer is configured in the maximal astable multivibrator mode with pins 2,3,6, and 7 tied together which yields a frequency of over 300khz.  No doubt Skynet chose this speed to keep heat dissipation and power consumption at a minimum.

Three of the 555 timer chips create a module using an advanced piggyback configuration to save space and dissipate power (see picture). Skynet’s technology guarantees that all three  555’s begin generating clocks simultaneously and in phase with each other upon power-up which is a level of precision humans are decades away from achieving.

The three 555’s form a triple modular redundant system (TMR) which can allow up to two chips to fail at a time, but then rejoin in lock-step when they recover.  The system is, of course, radiation hardened.

The duty cycle of the clock it generates is greater than 90% which is significantly more advanced than the 50% duty cycle commonly used today.

Regarding the origins of the 555… It has been speculated that Skynet sent a T800 back in time to 1971 to introduce this technology... Perhaps their goal was to guarantee a large supply of chips for their own future uses… There is a similar suspicion regarding the origins of the 741 operational amplifier...

One observation about the T800’s  high-performance 555 circuit is that it appears to emit a microwave “signature” when operating which could be detected by Skynet and used as a homing beacon. It also appears to gravitate and lock onto to some invisible force in space-time. 

I took a short video of the T800 Terminatot's 555 circuit exhibiting this phenomenon seen below.

Update:   There have been a number of questions, so I will keep a running list here:

Question 1)  A CPU speed of 300khz seems a bit slow for a Terminator, is this correct?

Answer: Good question! It is believed that the T800 uses a sixteen-way, superscalar 6502 with every imaginable optimiztion such as speculative fetch, out of order instruction retirement, and a high-bandwidth bubble-RAM interface.  One caveat is that five of the sixteen pipelines are dedicated to BCD instruction handling. And it goes without saying that it supports the undocumented opcodes...

Question #2) Why was the 555 timer used and not an Intel 8254 timer IC?

Answer: This is not clear, but maybe because of the limited 33% duy cycle of the 8284. Or maybe because it needs an external crystall while the 555 doesn't.  Or possibly becase the 8284 has a reset signal which the Terminator would never need as it never makes mistakes and would it ever need to be reset.  There are no erasers on the pencils in Skynet's offices for the same reason..

Question #3) How is the 6502 code stored within the Terminator?

Answer: It is highly compressed!  We have reverse engineered their algorithm which is capable of compressing a nearly unlimited amount of data into a single byte, but we are still working on the decomression algorithm.  Stay tuned!