Why does my toilet fill and slowly drain after showers?
Gurgling from the tub drain and/or slow draining showing could be from either a partial blockage in the drain line OR a venting issue. However, this should not raise the height of water in the toilet bowl assuming it's at the same floor level. The water level in the tub/shower would have to back up to an equal level for that to happen.
Unless of course you're flushing the toilet during or shortly after a shower.