SDMMC_test Reading is slower than writing？

[wx85105157]

I use arduino1.8.5，and use https://github.com/espressif/arduino-esp32

test the SD_MMC/examples/SDMMC_Test

2G SDSC 3.802281 Mbytes/S write and 1.070950 Mbytes/S read
4G SDHC 4.634659 Mbytes/S write and 0.851868 Mbytes/S read

test the 4G card in pc has 20Mbytes/S read。why Reading is slow？

rst:0x1 (POWERON_RESET),boot:0x1b (SPI_FAST_FLASH_BOOT)
flash read err, 1000
ets_main.c 371
ets Jun 8 2016 00:22:57

rst:0x10 (RTCWDT_RTC_RESET),boot:0x1b (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:1
load:0x3fff0018,len:4
load:0x3fff001c,len:808
load:0x40078000,len:6084
load:0x40080000,len:6696
entry 0x400802e4
SD_MMC Card Type: SDSC
SD_MMC Card Size: 1908MB
67108864 bytes written for 16832 ms
3.802281 Mbytes/S write
67108864 bytes read for 59760 ms
1.070950 Mbytes/S read
Total space: 1907MB
Used space: 64MB
ets Jun 8 2016 00:22:57

rst:0x1 (POWERON_RESET),boot:0x1b (SPI_FAST_FLASH_BOOT)
flash read err, 1000
ets_main.c 371
ets Jun 8 2016 00:22:57

rst:0x10 (RTCWDT_RTC_RESET),boot:0x1b (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:1
load:0x3fff0018,len:4
load:0x3fff001c,len:808
load:0x40078000,len:6084
load:0x40080000,len:6696
entry 0x400802e4
SD_MMC Card Type: SDHC
SD_MMC Card Size: 3796MB
67108864 bytes written for 13809 ms
4.634659 Mbytes/S write
67108864 bytes read for 75129 ms
0.851868 Mbytes/S read
Total space: 3795MB
Used space: 64MB