Why does an opto-isolated SSR specify a maximum number of cycles per second?
This is almost definitely for thermal considerations:

From the datasheet tables:

Note that the maximum frequency has conditions of V\$_L\$(max) and I\$_L\$(max).

There is a handy chart:



This shows the maximum operating frequency for various output power loading; for your case (18W total) you should be able to operate at up to 10 cycles per second. 

[Update]

The V*I curve above appears to be V(peak) * I(peak) rather than the RMS values more commonly found.

[Update 2]

Spehro made an excellent comment about the MOSFETs spending considerable time partially on, and that is definitely so:



At 10mA forward current, it appears to take almost 5 msec to turn on even at a very modest load.