Preparing game data here means building the command queue buffers, synchronizing system clocks (via ICMP or UDP timestamps), and performing a handshake that verifies all players have the same map checksum and data table hashes.
The progress bar crawled with the agonizing deliberation of a in heavy mode. starcraft 2 preparing game data
If you are experiencing this issue persistently, try the following verified community fixes: Preparing game data here means building the command