If proxying is enabled, which can be automatic depending on some renderers or manually forced with More > Settings > Renderer settings > (pick renderer) > Proxy Qobuz Tracks,
BubbleUPnP will buffer the playing track fully as fast as possible, with the renderer reading from BubbleUPnP at its own pace (which can be a tad slower).
With proxying enabled and depending on your network connection and Qobuz servers bandwidth speeds, it is possible a whole FLAC is read within a few seconds.
With gapless enabled, BubbleUPnP instructs the renderer of the next track immediately after the current track is playing, but it is up to the renderer implementation
to start pre-buffering the next track (from BubbleUPnP if proxying is enabled). It usually may do so 15-30s before the end of next track.
Note that 'Gapless control' enabled in BubbleUPnP for a renderer only applies for playing consecutive tracks of the same album, with the result of the album playing gaplessly if it is actually gapless (which the app doesn't know) and the renderer supports gapless properly.
'Gapless control' is not implemented as a general way to pre-buffer unrelated consecutive tracks.
As for "Qobus proxy title" I suppose you mean the setting displayed as "Proxy-Qobuz-Title" in German ?
There is a typo (that I have fixed) and it should read "Proxy-Qobuz-Titel" which is the translation for "Proxy Qobuz tracks".
'Track' is generally translated as 'Titel' in German.