My guess would be the SD card being the problem. SD cards aren’t that fast, even in best case scenarios and a raspi isn’t exactly a best case scenario. And besides, you’re probably sharing it with the OS. Try a USB HDD or at the very least a fast USB 3.0 stick.
(Also, Class 10 doesn’t say anything about read speed, only write speed that isn’t terribly fast either with 10MB/s. A UHS rating would give more indication about read speeds)