On my 1920x1080 monitor width of graphs Monitoring -> Graph is 1719 and stacked graph with 17 graphitems & 2 triggers (graph.get() can be found in the attached file) renders for 200 sec, that makes "quick look" completely impossible.
Rendering time increases non-linearly, please see results of http://example.com/zabbix/chart2.php?graphid=554&period=3600&width=... generating below.
As far as I can see — bottleneck is php: httpd consumes 100% CPU and mysql don't log any slow queries.

width (px) -> rendering time (second)
500 -> 2.1
600 -> 3.0
700 -> 4.1
800 -> 6.1
900 -> 8.3
1000 -> 22
1100 -> 40
1200 -> 60
1300 -> 85
1400 -> 108
1500 -> 140
1600 -> 170
1700 -> 200