The apps sit in the background and, in the case of Little Snitch, monitor and inform you about outgoing network connections. BlockBlock monitors for apps being installed that have the characteristics of malware in that they start when the Mac does and aim to run in the background.
I find the use of any malware apps concerning simply because of how such apps essentially destroy the experience of using a Windows computer, wherein developers play upon the fears of users and essentially take-over their computer with intrusive apps.
Little Snitch and BlockBlock are simply not in the same league, and commit none of those sins, but a question remains: Do they slow down a Mac?
And the answer is: Yes. But not by much.
Using Novabench and Geekbench 3, I benchmarked my MacBook Pro (2.8Ghz i7, 16GB RAM) when the apps were installed, and again when they were uninstalled. I ran each benchmark three times in succession, with all other apps terminated and Time Machine not running. The MacBook Pro was connected to a power source.
As always benchmarks are very unscientific but the results are:
Novabench with apps installed:
Novabench with apps uninstalled:
Geekbench 64-bit benchmark with apps installed:
— 3651 (single core), 13150 (multi core)
— 3726 (single core), 14104 (multi core)
— 3728 (single core), 14259 (multi core)
Geekbench 64-bit benchmark with apps uninstalled:
— 3936 (single core), 14312 (multi core)
— 3935 (single core), 14650 (multi core)
— 3953 (single core), 14761 (multi core)
In other words, there’s anywhere up to a 5-6% performance hit if you have those apps installed, although this will depend on what you’re doing with your Mac at the time.
I’ll leave you to make your own summary here: you might consider it worth the hit for the protection you receive, or you might not.
ADDENDUM: Some folks are no-doubt going to want figures with just Little Snitch installed, and not BlockBlock. Here they are:
Novabench with Little Snitch installed:
Geekbench 64-bit benchmark with Little Snitch installed:
— 3683 (single core), 14363 (multi core)
— 3612 (single core), 14424 (multi core)
— 3758 (single core), 14516 (multi core)