Why does disabling Fasttrack cut speed?

Question

System administration

I am not a system administrator and do not have any fundamental knowledge, this is for information.
Climbing in the Mikrotik switch, understanding what it is responsible for, and Google. I asked myself how to limit the speed or distribute it so that one host does not take away from another, by googling, the first thing I realized was to disable Fasttrack in the Firewall, since the connections passing through it ignore the queue tree and some other functions, which does not allow you to control the distribution.
Vopschem turned it off, and realized that now the Internet speed is not 500Mbps, but 140-150Mbps.
I also came across an entry about this in Mikrotik (or rather a picture)
https://wiki.mikrotik.com/wiki/Manual:IP/Fasttrack
Tell me the settings for dividing between 24 switch ports so that the overall Internet speed does not cut like that. ?
More precisely, I kind of know how to divide everything, but is it possible to do this without killing a large part of the speed?

Answer 1

A) This is a switch in the first place, albeit with the declared L3 functionality.
B) The L3-functional is carried out on the processor.
C) His processor is weak. Typically, a switch does not need a powerful processor, since all useful work should be done exclusively by the switch chip, with very rare exceptions.
But the gloomy Mikrotik genius decided that it was also normal to forward traffic to the switch on the switch, and began to push it as an "L3 switch" although it is not one (nevertheless, the term L3 switch most often means a device capable of routing IP to a switch -chip, not the way Mikrotik did it)
Fasttrack in Mikrotiks has been enabled by default for a very long time, just like the long-standing appearance of the CRS-series of overgrown home routers. And before, it helped to show routers with gigabit ports and a crappy CPU almost-gigabit speed in standard NAT scenarios, which without it barely gave out 200-300Mbps (such as RB750G (R2)).
It greatly helps weak pros perform NAT and complex filter rules by replacing the full packet path through the system kernel (classification, packet filter pack, routing, interface queues) with a short FASTPATH ​​path that can do something (for example, NAT / PAT). Naturally, in this case, everything that is not supported by FASTPATH ​​itself does not work. Other manufacturers of small routers have their own tricks for this purpose, such as NAT Offload, which, in principle, works in a very similar way.
In the end, no one promised gigabit routing on this piece of hardware, the manufacturer's page shows routing with 25 rules (which roughly corresponds to one NAT translation rule) - up to 700 Mbps, and if you add a queue to this and a dozen more curved firewall rules, then 150 Mbit is quite the expected result. By the way, in principle, it will not route more gigabit under any circumstances - there is a link to the CPU from the switch chip - 1 Gbps.
In a normal switch, it will give out the prescribed 25 gigabits.
on some models, you can limit the speed even without queues, with hardware shapers / polishers on ports that will not affect the performance of the device as a whole: https://wiki.mikrotik.com/wiki/Manual:CRS1xx/2xx_s...
Naturally, slicing will not work there more precisely than a port / hard queue, but this is quite expected from a budget switch.

Answer 2

Because the switch has a low power processor. By disabling fasttrack, you force this processor to process all passing traffic. The processor cannot cope with the load, therefore, a bottleneck is formed and the throughput drops.