A 5i20 ( with firmware SVST8_4 down-loadable) can have 8 encoders and 8 pwm and will have 24 I/O points still available.
Since you are probably not going to use the 7i33 card ( DAC and encoder filtering )
you could set this same firmware to have 6 encoders and 6 pwm with 36 I/0 points.
but don't you need an encoder for MPG jogging?
Other combinations are possible - Mesa stuff is very configurable.
I assume you mean processor speed. More important is latency.
Not all computers have good latency you really need to check the actually equipment you want to use. there is a latency test that comes with EMC.
Here is a user made list of equipment with latency figures:
EMC Documentation Wiki: Latency-Test
Now after latency is good then more processor speed may help the response to non realtime stuff like the screen controls etc.
There are some videos of robot arms with EMC:
EMC Documentation Wiki: Videos
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