Well no easy or fast answer is possible , because
ALL OF THIS matters :
https://www.duckware.com/tech/wifi-in-the-us.html
The classic drivers of Mikrotik have great unique features, but act with (too) small transmission blocks, and waste al lot of transmission time that way. Maximum data rate is around 360Mbps for an 866 Mbps interface rate. That is in ideal conditions, in one direction, free field without interference of anything else. Just because of wifi contention-mechanism overhead.
The off the shelf "wifiwave2" wifi driver lacks all those features, but uses proper transmission block (AMPDU) sizes, and can get up to 600 Mbps in the same ideal closeby and lonesome conditions.
Many more limitations are to be considered. TCP protocol congestion avoidance can play a major (bad) role in this. That hotspot/portal will also contribute to the speed reduction.
viewtopic.php?t=188338 , follow the "packet transmission overhead calculation" link. for the above theoretical maxima.
But indeed 35 Mbps is slow. Finding the root cause is not simple ... in a shared medium with the wifi ether waves, there can be many causes. If the reason is in the wifi link, what is not a given yet.