Slow performance: PB report server

Dear all,

I use one server with report server install on it (16GB Ram, 16 CPU ). 

my report connected to SSAS cubes server ( 32 GB Ram, 16CPU).

My Cubes size is less than 3 GB.

But still, my reports keep in loading phase ( visuals loading slow m but the portal open fast )

Can anyone give me suggestions please to improve the performance?

regards,