It's all a bit vague really! I have constant issues with SSRS (large) reports. I'm thinking likely an issue with the rendering - you don't say in which format you're outputting the report, have you tried changing this?

In no particular order I'd suggest you hunt out all the various logs to see if you can get a clue there. You could try rebuilding indexes etc. in the various databases including the tempdb and reportserver tempdb and/or update stats etc.

the async network may indicate an issue communicating with the client e.g. potential latency - however if the report pages then it may mean nothing.

Try limiting the result set until the report renders, try 10k rows, less if required.

about the best I can offer to be honest