Are the two devices connected directly or via a relay? If it’s the latter, there may be some bandwidth limits on the relay side in place. Also, what is the hardware? Is it something decent or rather on the slower side?
Assuming the network is the bottleneck and that you can improve things by having more data in flight, which is not guaranteed, the things you want to look for are pullerMaxPendingKiB (how much data we ask for, from the receiving side, default 32 MiB) and maxConcurrentIncomingRequestKiB (how much data we try to serve concurrently, on the sending side, default 256 MiB). If your data is lots of tiny files there are a myriad of different bottlenecks and the network won’t be it.