Below will be the short and long timings of two seperate servers, one is running on 1.8 Spigot, the other on 1.8.8 Spigot.

1.8
Long: http://timings.aikar.co/?url=12001403 Hoppers @ 1.16% of a tick
Short: http://timings.aikar.co/?url=12001422 Hoppers @ 1.80% of a tick with 2,813 active, 9,895 idle.

1.8.8
Long: http://timings.aikar.co/?url=12001377 Hoppers @ 77.46% of a tick
Short: http://timings.aikar.co/?url=12001405 Hoppers @ 67.98% of a tick with 1,486 active, 4,184 idle.

Undeniably, hoppers wreck an incredible amount of performance compared to pre-1.8.3 builds. I've come to the conclusion that it's due to this commit (source: "↳ hopper-check (Removed in this 1.8.3 commit from March, 8th 2015)" - from spigot.yml wiki), which made the hopper-check setting in the spigot.yml defunct. I would rather the various hopper contraptions not work while still keeping my server afloat than them not working due to the unbearable lag they bring.