Imo, the speed at which merit is given should not be of great concern, rather the quality of post being merited. However, there a some factors that can lead to successive merit awarding withing an infinitesimal interval. They are;
1. Bookmarking of quality posts to be merited when merit is lacking. Someone who had already noted or bookmarked posts to be merited does not necessarily need to re-read the posts before meriting it as said by OP.
2. Someone might need all the replies in a page of a thread before he/she will start meriting. This can be common with merit sources on especially a merit cluster or giveaway thread. It can also be someones way of meriting. Most times I do this when I have a few smerit to give. O can hastily give it out and later find a better post on same thread, so I usually read through before meriting.
3. A user who is in a campaign, has a personal project and manages more than one account can have very tedious work in the forum.  Therefore may decide to have a forum time table, that between a certain time will be for posting, another time for post counting, another for meriting, another for marketing and maybe for plagiarism hunt etc. So, when the time is for meriting, you can see them coming within seconds.