The reasons already described by @stefanbenten SQLite has its limitations, but slow is not one of them. As long you don’t need multiple connections over a network it is a great choice.
The main performance problem is the chosen dbchema. At the last time i looked in the db i saw that near 30% of the data are used only by the id of one satellite which is written over and over in one table.
The db structure need a bit of normalization, not to much or everything, because normalization could decrease performance, but in some points it will result in a smaller and faster db design.
Second it should be checked if everything in this database is really needed.