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Although the 400 Series cameras work with any Intel or ARM processor, your 4790 processor is a 4th Generation chip that was considered to be the minimum requirement for the first RealSense cameras in 2014, whose technology was simpler than the 400 Series of today. Intel chips are now on the 8th Generation. So your old CPU may very well be creating processing bottlenecks that result in lag when using the camera on high settings.
The USB lag that you report is reminiscent of the RealSense R200 camera, one of the original RealSense generation, that would sometimes have applications freeze when running them at 60 FPS because the USB port was overwhelmed with bandwidth, and the solution was to drop to 30 FPS to reduce the bandwidth being sent to the port.
In regard to your questions:
1. As far as I know, there are not benchmark charts available for camera performance on different hardware specifications.
2. Support for hardware timestamps is not yet available, and system-time is being used until then. There is currently no date available for when hardware timestamp support will be released.
3. The RealSense community can likely give better recommendations than I can about good USB monitoring utilities.
I've made some more tests on systems of a less performance.
1) Lenovo T450s laptop (i5-5200U 2.2GHz), windows 10, USB3. The Viewer has capped maximum resolution settings to 640x480, within these limits no problems observed.
2) Desktop i5-3570 3.4GHz, windows 10, USB3. Although the Viewer allowed maximum resolution settings, it has put limits on FPS, only 6 frames per second. Within these limits I had depth sensor stream heavily lagging, USB SCP overflow messages present.
It looks like Viewer is doing some estimations in order to pick its limits. As windows resource monitor shows, the CPU usage is not overwhelming, which could mean it's bandwidth limited.
I'm going to dig into the source code to find out what kind of estimations Viewer makes. Also I think about buying PCI-e USB3 extension board.
A PCI-e USB expansion board probably would not make much difference if the bottleneck is in other hardware components such as the processor and graphics.
It is worth noting that that a 5200U (5th generation processor) that is newer than the 4th generation machines you mentioned earlier would have worse performance than 3570 (3rd generation). It is likely that the faster GHz of your 3rd generation processor, and perhaps a better graphics component in the desktop machine than the one in the laptop, help to provide greater capabilities. For example, my main work machine is a laptop with a 6th generation processor, but it runs very slow compared to my desktop machine with the same generation processor because I had a dedicated graphics card in the desktop.