This problem is solve.
SD Formatter formats Flash so that the performance of a Flash can be pulled out to the utmost.
When formatting a Flash, it recommends strongly using not the format tool attached to an operating system but this SD Formatter.
Although the format tool attached to an operating system can generally format various recording media containing a Flash, It may not be optimized by the Flash and performance may fall from the case where SD Formatter is used.
so, procedure is below (now "SDFormatter" is windowsOS only).
01. Download "SDFormatter" from this link -> HERE
02. Install "SDFormatter"
03. Enable "USB Debugging Mode" from Developer Options in settings.
04. Connect your tablet to PC (USB).
05. Then, tap the "USB Strage Mode" to connect to PC.
06. Backup data on INTERNAL_SD card.
07. Run "SDFormatter".
08. Specify the drive letter of INTERNAL_SD in SDFormatter and click "(Option)" button.
09. "FORMAT TYPE" select "FULL OVERWRITE" and set "FORMAT SIZE ADJUSMENT" to "ON" and click "OK"
10. Now click "FORMAT" and wait for the procedure to complete. (Can take approximately 1 hour 40 minutes)
11. After format is completed you can recopy the backup files back onto the INTERNAL_SD
12. HAVE FUN !!
probably this act is effective in all the China tablets.
this storage faster effect is in all the tablets I own.