The first time in the MATLAB session, 'gpuDevice' would search for available GPU devices and store the information for that session. This information is reused throughout the session. The scanning might take some amount of time depending on the system.
Another possible reason might be the license checkout time. If you have multiple license files, or multiple network license files, then the time taken to verify the license while gpuDevice is invoked the first time might be considerable.
To fix this, remove any invalid files that are presented as part of the following output:
>>feature lmsearchpath
This lists the set of local license files present. Delete the outdated ones and retain only whats required.
Also, check the environment variables 'LM_LICENSE_FILE' and 'MLM_LICENSE_FILE' and remove any old references. Please refer - https://www.mathworks.com/matlabcentral/answers/93105-what-are-the-lm_license_file-and-mlm_license_file-environment-variables-and-how-do-i-set-or-check-fo
