Adding external subtitles would freeze the player, even during download

The download of external subtitles should be performed in the background. However, for some reason, it freezes the player even during the download.

ie.

[DBG] net: connecting to dl.opensubtitles.org port 80
// ... stuck here for a minute
[WARN] failed to add

Expected:

The player should freeze during the subtitles download.