Have a problem with RB Metal

I have one RB Metal 2 SHPn with 15db omni antenna in the garden of a hotel.
I do not have a problem with the coverage, its just fine… at the very beginning it was working fine… I was the only one connected with my laptop.
But now, we have more that 300 guests…
For each address that DHCP would give to connected clients I have a queue to limit 2M/2M
My problem comes when more than 30 clients are connected…
even if no “big” traffic goes in or out, the ping start going up to 200 ms and more… from time to time during the day.
At midnight when there are a few clients (10-20) its normal again…
when I have more than 30 clients on the AP, the CPU is about 20%, which for me is ok and I believe its not an issue…
I tried to change the channels, going to a better one a couple of times… its the same.

What device should I use instead of Metal?
I need something that works fine for multi sessions.

is one hotspot or only one access-point?

I havent mention anything about Hotspot. Its just one AP

Try new wireless package. Could help a bit. Anyway I would suggest to use more sectors and spread the stations between them.