Currently Software Center will use WMI to query all available software for the computer, then it will ask the management point for user assigned apps.
If your computer is a bit slow/old the wait time checking WMI can be painful. You can check this on the client log SCNotify_MONASH@%username%_1.log
Please vote for my Uservoice - https://configurationmanager.uservoice.com/forums/300492-ideas/suggestions/39082435-speed-up-software-centre-searches-by-returning-use
If your SQL is slow you have another problem.. You can check this on the MP UserService.log - Search for GetFilteredApplications.
SQL Fixes:
Set the right compatibility level - https://support.microsoft.com/en-au/help/3196320/sql-query-times-out-or-console-slow-on-certain-configuration-manager-d
Reindex right - https://www.scconfigmgr.com/2017/01/03/configuration-manager-sql-database-maintenance/