Multiple possible reasons: The retrieval from a database is slow There is lots of IO Your code is slow The system is busy with other things And don't forget front-end performance

%$ vmstat 1% procs memory swap io system cpu r b swpd free buff cache si so bi bo in cs *|dd1111|us sy|* id wa 5 0 121248 367520 767824 3080668 0 0 11272 0 1593 5540 *|dd1111|74 22|* 3 0 5 0 121248 290784 767960 3082980 0 0 268 0 1555 5381 *|dd1111|77 20|* 3 0 5 0 121248 238340 768132 3084336 0 0 1364 21160 2263 6815 *|dd1111|70 21|* 3 7 6 0 121248 170772 768300 3087100 0 0 1652 0 1802 8540 *|dd1111|71 25|* 4 0 CPU is (close to) fully in use: *|dd1111|74 + 22|* ➔ your *code* is slow.

%$ vmstat 1% procs memory swap io system cpu r b swpd free buff cache si so bi bo in cs us sy id *|dd1111|wa|* 0 1 121248 50776 314796 4156000 0 0 245800 0 3018 5394 1 11 71 *|dd1111|17|* 1 0 121248 52112 315108 4236692 0 0 243672 0 3107 5552 1 10 71 *|dd1111|18|* 1 1 121248 50148 315256 4318268 0 0 243096 44 3146 5463 1 11 72 *|dd1111|17|* 1 1 121248 52120 315484 4396356 0 0 243784 0 3006 5419 1 10 72 *|dd1111|18|* *wa*it is > 10 ➔ IO is the bottleneck.