Slow Windows server web performance (high TTFB)

I am currently looking at an issue with one of our Windows Servers, whereby the exact same web app running on another server is fairly fast, but on the problematic server there is a long "time to first byte" on several files (roughly 7-8 seconds of "Waiting (TTFB)"), particularly the larger ones. This results in the page taking 10 seconds+ to load versus >1 second on the other server.

Download speed of the ~800kb content once the first byte is received is quick, around 20ms, so I think I can rule out network issues.

I performed a hard refresh of the page while running a few parameters on Performance Monitor and noticed that it appears to not be using much CPU, and the % disk time is fairly low and spiked during the load time whereas the same parameters on the server that loads the content quickly shows much more activity, with the spikes reaching 100% or close to, and finishing the operation quickly.

I've been pulling my hair out trying to work out what the issue is here. CPU load is low, as the diagrams show, so I do not believe that it is that the processor. There is roughly 8gb of the 16gb of RAM free on the server, and I can't think what would be causing the load on the hard drive to not hit 100% and just return the files quickly.

I must state that I do not usually work with these kind of issues with a server, and am simply a junior developer, so it's likely that I might have missed something simple or am coming to the wrong conclusions.

Any ideas or thoughts are much appreciated!