Did speed testing for relays.   Results:

RES55 - 1700Hz on 50% duty cycle - 300us/600us ON/OFF delay

RES64 - 1600Hz on 50% duty cycle - 200us/400us ON/OFF delay - Why this is slower? ;-)

On 1200Hz both types of relay works very stable with clean charts, on max speed contact bounce become visible. Signal decreases to zero so slowly because of Open circuit.

Schema is very simple:

Set square signal to transistor, and get signal from one relay output: