It is not very surprising that it takes a long time for synonym to be approved (=get enough upvotes) or removed from the list (=get enough downvotes). There are not many users who can vote for tag synonyms. Especially for tags which are not used very frequently there are not many users which have score $\ge5$ in the particular tag. (This would mean the the review queue you proposing would be either empty for many users or it would show them synonyms on which they cannot vote.)
As far as I can say, at the moment creation of tag synonyms on this site works like this:
- If somebody has a suggestion for a tag synonym, they usually post on meta, where this synonym is discussed. (This can be a separate post, if the synonym needs more extensive discussion, or an answer in the designated thread.)
- Here the users upvote/downvote suggestion.
- When the voting suggest that there is some kind of community consensus, then some user suggests the tag synonym. If this is mentioned on meta, it increases probability that some users will vote on the synonym. In the case that there is clear consensus, this is often done by mods. (Only one vote from a moderator is needed, as their votes are binding.)
I see several advantages of this system. More users can vote. (For voting on meta there is no restriction on the score in the given tag.) And they can also post comments or answers explaining why they think the particular synonym would be useful or the opposite. (So other users see more information than just the total score.) So I think it is a good system. (And I prefer this system to creating a new review queue.)
If you see that some tag synonym is pending for a long time, you can bump the post where it was discussed to get more attention to it. Or you can mention it in the tagging chatroom.
It seems that several of the tag synonyms which are pending at the moment were discussed on meta. (But very probably not all of them.) I will post a list of posts in a separate CW-answer.
Two more comments on your original post: "Even worse, there are synonym suggestions with only one vote since September 10, 2014". As far as I know you do not see the total number of votes. For tag synonyms, only the total score (i.e., number of upvotes minus number of downvotes) is shown.
You wrote: "To speed up these things..." I do not think that it is a good idea to be in a hurry for creating tag synonyms. We have reviews for closed votes, reopen votes. But results of decision to close or to reopen is relatively easily reversible. Removing tag synonym seems to be a more difficult process.