Thunderbolt bridge between iMac Pro and M1 Mac Studio

I have been experimenting with files transfers via thunderbolt bridge between my iMac Pro and M1 Mac Studio. When I ping them using iperf3, I see a bitrate of around 20 GBits/sec. I’ve got them connected via SMB. When I copy a file from the iMac Pro, the activity monitor says it’s sending data at 4 GB/s. However, my Mac Studio is only receiving data at 1 GB/s. Sending from Mac Studio right now at 2.5 GB/s , and iMac Pro receives at 1.75 GB/s. I’ve tried rsync and ssh and they both slow the transfers down to 200 MB/s or less. My first question is - I thought I’d be reaching speeds of 40 Gbits/sec? Why am I pinging at 20 Gbits/sec? Second question - if the iMac Pro is sending at 4 GB/s, why is the Mac Studio receiving data at 1 GB/s? I know its internal SSD can write at higher speeds than that. Thanks for any advice.