Most likely reason for slowness.
1) 3ds hardware simply can't take advantage of class 10 write speeds. it comes with a class 4. IF class 10 really helped, they would come standard with them.
2) installing cias requires more than just copying it also has to be decompresed. in this case, o3ds is slower than new.
3) has to read and write to same device. this slows it down too.
4) the emunand hook, if using it, probably also slows things down.
Those who complain about slow install times.. is it an old or new 3ds?
If the game is available on the freeshop it's usually a good idea to get it there. this bypasses the cia conversion step.
BOttom line is you are limited by the 3ds hardware if you have a decent class 10
Extreme IS faster then ultra. specifically, the U2 on the extreme means it's faster than the U1 on the ultra.
But the 3ds, and especially the old 3ds can't really take advantage. that's why it came with a class 4.
Cluster size may also be an issue. The proper formatting tool for sdcards is NOT windows. it's the official sd card formatter. (
https://www.sdcard.org/downloads/formatter_4/)
The exception is if it's greater then 32 gb. in which case you must format as fat32 using a different program. this WILL cause slowness though.