Ctrl-s is the composer's religion XD

A 128-sample buffer wouldn't account for 3/4 of a second. Something else is causing the unresponsiveness, but I have no idea what it might be, as I've never observed it.
 
I have, however, experienced keypresses not having any effect, e.g. hitting "W" to rewind. That has always turned out to be a focus issue, e.g. a plugin's UI has focus and "W" means something special to it.

Anything like this (channels in Saffire Pro 26):
 
Note: The Allow Arm Changes During Playback/Record option is disabled by default. When enabled, SONAR must keep all hardware input ports open so that it can dynamically arm tracks on the fly. With some drivers in WDM mode, this can add significant overhead if there are many input devices enabled in Edit > Preferences > Audio - Devices. This may lead to more frequent drop outs. When using this option in WDM mode, it is recommended that you disable all unused input drivers in Edit > Preferences > Audio - Devices to minimize this overhead. Disabling unused inputs is not required when using ASIO mode. (from help)