I wouldn't dare to say it so simply.
As long as your bridge port has the "H" flag, it is hardware offloaded = switched. That does not affect the speed. However bridge without HW offload must be processed in CPU and that will definitely affect the speed.
If you use vlan filtering in bridge, all HW offloading goes away. HW offloading can be also disabled by several other featues or can be disabled manually on specific ports.
As long as you maintain HW offload, this chart applies: https://i.mt.lv/cdn/rb_files/RB750Gr3-e ... 152443.png
That clearly shows that all switched ports are connected to CPU with a single 1Gb line, while non-switched ports share second 1Gb line.
If you don't have HW offload at all (i.e. disabled switching), this applies: https://i.mt.lv/cdn/rb_files/RB750Gr3-d ... 140316.png
That is much more interesting, isn't it? ports 1,3 and 5 share one line while ports 2 and 4 share second line.
Now lets apply it to your example: Ports 2-5 bridged together. I assume port 1 is not switched (probably WAN uplink)
As long as HW offload is active, CPU will give dedicated 1Gb line to port 1 and another 1Gb line to ports 2-5. Communication between ports 2-5 will be wire-speed but they will share that uplink to CPU.
You could say, that this is EXACTLY SAME as if you remove bridge and connect external switch, however, that applies only under condition that you connect the switch to port 2 or 4 while rest of ports 2-5 will remain empty. If you connect anything else, it will not be exactly same as some ports will share the line to CPU.