Hi,
Simply, As you know every connection need both send and receive. If You limit the client (which have receive more than send) it will affect on the speed. for example , when you open google.com and write in search filed “Mikrotik”, this word have to send to the server to reply to your search. So the send speed is also important.
If I were you, I set the profile limitation like this:
Instead 25k/400k ----->> 200k/400k ---->> send is half
let lets say that I have 12mbps download and 1Mbps upload and 30 active 400kbps download users. wouldn’t 200kbps upload be too much for them. what do you suggest?
As I said you can use burst. Read about burst in Wiki and you will see :
for example if you set max upload to 25k, burst threshold to 50k ,burst-limit to 100k and burst time to 8s,
the router will check every 8 seconds and check if the user reach the threshold, see if the bandwidth is free let him to reach to burst limit.