Yes Braiins software takes 2.5% of your hashrate by periodically switching to what is called a DevFee pool and yes when it does you are not mining elsewhere. Stratum has an optional feature which can be enabled called Nsub that lets a miner seamlessly switch work to a different pool/coin without rebooting and you having no indication when it does that. The DevFee config is hard coded into the Braiins (or other 3rd party) software. It is also often considered a security hole because you have NO idea what it is doing and when. In this case you are knowingly allowing it to switch to the DevFee pool and you have to trust Braiins for that to be all it does.
While you can use a tool like Wireshark to find the IP of the pool and then block it, doing so will result in the miner shutting down until it is again allowed to 'phone home' so Braiins (or Vnish et al) get paid for you using their software.
Ohh okay, so it's true. Yes, you're right, of course you don't know what Brain or the software is doing. Is it now 2.5%, or maybe 3 or sometimes 5, hmm, no idea
I don't think this is how braiins works, I have it running on a few of my S19 types, where I split the pools to mine on. For example I direct a small amount of hash to solo mine while the rest is one a pool like braiins. You can mine on several pools with that o/s simultaneously without issue, and the 2% fee is basically an invisible pool the miner points to.
I ran several pools (think it was 5 pools all being mined on the same ASIC), and even solo mining other coins like BCH - it had actually found blocks. I see the 2% loss in hashrate to make it more efficient and then can allocate a small portion to solo worth while. I have found no indication in loss of mining, in fact I view being able to make the miner more efficient worthwhile, and if mining on braiins with their o/s also gives an equal bonus by no pool fee so evens out.
What I am now trying to test is if the "main pool" somehow is better to solo mine than say "pool 2" or whatever down the list.