Yes, just set all the ports you wants to be part of the switch except one to be the slaves of one port, If ether1 is connected to the wan and ether2 through ether24 are to be part of your switch, just do this:
/interface ethernet
set ether3 master-port=ether2
set ether4 master-port=ether2
...
set ether24 master-port=ether2
/ip address
add interface=ether1 address=<your WAN address>
add interface=ether2 address=<your LAN address>
Yes, you should have a little better as CPU frequency is 400MHz for RB750GL and 600MHz for CRS125.