[ericbb]

With the C-c issue specifically, I don't think that emulator throughput is the most likely culprit.

It could be that the emulator is not handling inputs fairly: maybe it tries to process all available input from the pseudoterminal before processing the next batch of keyboard input. Or it could be that the pseudoterminal (kernel driver) is not configured to send the signal as expected. Or it could be that the process you're trying to interrupt is not responsive to the signal.

I maintain a terminal emulator that is not optimized at all and I just tested interrupting a process that was dumping one gigabyte of text to the screen. The interrupt was handled instantaneously.

[JdeBP]

It isn't the most likely culprit. The mosh people point out that the place where people hit this is with a SSH session to another machine. Where the data are actually building up is the SSH connection. It's not a problem with the terminal emulators at all.