In some cases, resource intensive desktops like GNOME 3 or more in general desktop environments with 3D graphical effects may lead to slowdowns in NoMachine sessions.
3D graphical effects (e.g. shadows, animations, fading menu etc..) perform better on hosts with a capable GPU (Graphics Processing Unit) but are made available also on those machines without a GPU. In this case software rendering is done on CPU instead than on GPU. This process is CPU-intensive and does require sufficient CPU power for a good experience.
Slowness with 3D desktop effects can be due to CPU overload during the software rendering process and happens independently from a NoMachine session.
If a slow 3D rendering affects NoMachine connections to the physical display of the remote host, this is due to the display own capabilities. In this case a possible solution is using a lightweight desktop instead than a 3D desktop. A lightweight desktop like Xfce or Mate is less resource intensive and will help to reduce the CPU load. You can find some examples for installing and running an alternative desktop here:
All the suggestions above apply also to NoMachine virtual desktops. In case of virtual desktops, adopting a server host with GPU and 3D drivers could be an alternative, this would allow to use hardware acceleration. Before proceeding with this last option, however, you need to be aware of possible issues with NVIDIA and AMD proprietary drivers: https://www.nomachine.com/AR09L00814.
When the server host has a GPU, it could be useful to enable the VirtualGL support. OpenGL applications such as GNOME 3 makes software rendering even if there's a GPU. The VirtualGL library redirects the 3D primitives to capable graphics hardware so it makes the OpenGL application able to use the GPU rather than making software rendering. To enable VirtualGL support for the whole desktop or for a specific OpenGL program only please read here: https://www.nomachine.com/AR05P00982.
For virtual desktops running on Red Hat Enterprise Linux /CentOS v. 7.5 or later read also: https://www.nomachine.com/AR01Q01012.