The main user platform of SWAN currently is Alma Linux 9 (Supported since LCG 105a), so we highly recommend migrating to that new image, as it is maintained.
Furthermore, you may need to try to start your session a few times, if you still intend to use the CentOS7 image, as it needs to setup the environment from the beginning and that requires additional time, where as the environment is already set up for the Alma Linux 9 one (because it is the default).