The assets being used in the past were not nearly as High Quality as the ones used now, this map is also much bigger and the models are much higher quality/droids function much better and we have many more customs scripts/addons than we used to. (remember source doesn't render what you can't see, therefore the higher quality meshes/models cause more strain being rendered than your 1000 pixel play dough white model from 2018-19.)
The technological advancements on an engine as old as Gmod's have not been good when it comes to performance. Gmod will likely never recieve an update to allow for full utilization of the server boxs people pay for. Therefore no matter how much the box is upgraded, server performance will maintain a steady line rather than improving noticeably.
That's what i know, im sure there are many more reasons and network issues at play that Xaze can touch on much better than myself. This is just observations from my time on the server.