WHR-HP-G54 deadly slow and weak

Hi guys! I'm helping a friend of mine set-up his old Buffalo router in a 100 sq ft space used only on friday nights for private parties (friends' birthdays, etc.) We are talking about 60-70 people connected at once, mainly on smartphones - meaning IM apps and light browsing at most. Link speed (LAN internet) is 64mbps up / 32mbps down, achievable 99% of the time when connected directly to a PC. The space is free of other signals, because it's deep in a basement, 2 levels below the ground.
I'm...

WHR-HP-G54 deadly slow and weak