If you find QuantumATK slow to start, there can be several reasons. Most of them can fixed, however, and once things are set up properly, NanoLab should fire up in a few seconds only. However, keep the following things in mind:
Don't make projects too large. The more files there are in your project, the more work for VNL to both start up and close down. In particular, never turn your home directory or root directory into a project - if you prefer to just browse the file system, just use "Classical mode" instead!
If your project contains VASP files and you don't have a ATKVASP license feature (which can be obtained for free for any purchased license), startup will be slow if you have the environment variable QUANTUM_AUTOMATIC_DISCOVERY turned on. Remove this variable, and ensure the environment variable QUANTUM_LICENSE_PATH points to your license server instead. If you have a standalone license file, you also don't need QUANTUM_AUTOMATIC_DISCOVERY set. Older versions of VNL would however set this variable automatically, so it may be activated without your knowledge.
Sometimes startup is slower, sometimes faster. This depends on caching in the operating system.