There could be a few things going on. First, MakeMKV's speed control isn't strictly a thing that only makes reads go faster. Between layer switches and around the edge of the disc, it purposely slows down to improve the chances of a successful rip. Second, sometimes discs can be dirty in ways that aren't really visible to the Mark 1 eyeball. Sometimes, things go slower as a result. Third, when ripping a disc, the optical drive is usually running in a Constant Linear Velocity mode (
https://en.wikipedia.org/wiki/Constant_linear_velocity). This means the disc is rotating at a constant rpm and consequently the outer edge of disc passes by the laser head more quickly than the inside of the disc. This means, all things being equal, the read speed will vary as the laser head changes position. Also, optical discs can have between 1 to 4 layers of data, meaning the laser head might make the trip from inside to outside to inside multiple times when ripping a movie.
That's all a long winded way of saying, if you set the speed control string, then it is working. And MakeMKV is doing the best it can with each piece of media.
As a side note, MakeMKV can use more than one optical drive at the same time. So, if you have a lot of media to rip, the fastest way to get through it is to get more than optical drive.