I have experience with 3 devices:
1. LG Optimus One, which had 170MB internal memory, so everything was on SD Card. It booted fast, but then I had to wait 2 minutes for SD apps to appear and then a few more seconds for the SD to be fully usable.
2. Galaxy S3 mini, which had 8 GB internal storage + 8GB SD card. It booted fast, all apps useful, but it lagged until the SD was mounted and any apps loaded before had horrible crashes, or redraws, you just had to wait like 30 seconds for the SD to mount and 10 more to be usable.
3. Moto G. No SD, it boots, it works.
Depending on the device and memory used, the SD might be boosting (it did on the P500), but it slowed down my S3 mini, and I only had my music on it.
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