You can also have a worldbuilding in which there are technical difficulties on isolating or filtering the signals across a neurosynaptic link, so most people (other than the most resourceful hackers) have no choice but to expose themselves with a direct neurosynaptic link.
Galvanic isolation of high-speed digital electronics today is already fairly difficult. Imagine adding optical isolaton all the data ports on desktop computers, including the high-speed ones like 20 Gbps USB or Thunderbolt, it's entirely possible but difficult and expensive, sometimes with compatibility issues. USB 2.0 High Speed is a notorious example, it's difficult to isolate (until it was recently solved by some new ASICs from TI and Analog Devices) not because of any inherent technical problems of data transmission, but because its signaling and protocol are not designed with transparent repeaters in mind. Thus, galvanic isolation is only used in highly specialized applications. As a result, a USB Killer can easily destroy most PCs because the signal is often wired straightly into the CPU (SoC).
One can only imagine the difficulties of doing the same for a neurosynaptic link in a Sci-Fi world. For example, in Ghost in the Shell, ICE is in widespread use, meanwhile isolating firewalls do exist but they're rare, mainly used by intelligence agencies. They are also disposable devices and would be completely destroyed after an electrical overstress (not unlike real-world galvanic isolation...) Further, one can use bandwidth limitation to rationalize the "the mental equivalent of walking through treacle" part of your plot.
Of course, as you've pointed out, as computers operate much faster than the human time-scale, the argument of bandwidth is not really that convincing.
Galvanic isolation of high-speed digital electronics today is already fairly difficult. Imagine adding optical isolaton all the data ports on desktop computers, including the high-speed ones like 20 Gbps USB or Thunderbolt, it's entirely possible but difficult and expensive, sometimes with compatibility issues. USB 2.0 High Speed is a notorious example, it's difficult to isolate (until it was recently solved by some new ASICs from TI and Analog Devices) not because of any inherent technical problems of data transmission, but because its signaling and protocol are not designed with transparent repeaters in mind. Thus, galvanic isolation is only used in highly specialized applications. As a result, a USB Killer can easily destroy most PCs because the signal is often wired straightly into the CPU (SoC).
One can only imagine the difficulties of doing the same for a neurosynaptic link in a Sci-Fi world. For example, in Ghost in the Shell, ICE is in widespread use, meanwhile isolating firewalls do exist but they're rare, mainly used by intelligence agencies. They are also disposable devices and would be completely destroyed after an electrical overstress (not unlike real-world galvanic isolation...) Further, one can use bandwidth limitation to rationalize the "the mental equivalent of walking through treacle" part of your plot.
Of course, as you've pointed out, as computers operate much faster than the human time-scale, the argument of bandwidth is not really that convincing.