Unfortunately spawners/farms are a huge burden on server performance. Caps on these things are pretty much the very first step to avoiding having a laggy server. Minecraft is a poorly coded game (though slowly getting better since Microsoft bought it, it seems), and having a lot of entities bogs it down. Sometimes entities glitch out and clump together, which amazingly can lag the entire server massively. It's so stupid.
There might be some room to increase the cap, but I can't help but not want to. (A few years ago I tried, and there was no perceptible difference in spawn rates, strangely.)
It's also worth noting that MC's mob spawn mechanics are notoriously unreliable on large enough servers, cap or no cap. There are endless complaints on the internet about this crap. There's even bugs that certain co-ordinates or biomes will fuck with spawn rates. It's a messy game.