littlefs on SD/eMMC read/write performance is poor

Hi,

Utilising the littlefs filesystem on SD/eMMC is providing read/write speeds of approximately 50KiB/sec (calculated using dd command). I've tried optimising the menuconfig settings to improve it, including options such as below but still the performance is lacking:

- Turning on CONFIG_MEMCPY_VIK gave slight improvement
- Setting USEC_PER_TICK to 1000 or below gave most improvement, however at the detriment of other aspects of the system. The fastest speeds observed by adjusting this was ~370KiB/s write and 700KiB/s read though overall this was unacceptable given the effect on the rest of the system.
- Adjusting LittleFS parameters (e.g. CONFIG_FS_LITTLEFS_PROGRAM_SIZE_FACTOR, CONFIG_FS_LITTLEFS_READ_SIZE_FACTOR, CONFIG_FS_LITTLEFS_BLOCK_SIZE_FACTOR, CONFIG_FS_LITTLEFS_CACHE_SIZE_FACTOR, CONFIG_FS_LITTLEFS_LOOKAHEAD_SIZE
- Ensuring SD/eMMC DMA read/writes are enabled
- Setting MMCSD_MULTIBLOCK_LIMIT to 0

Details of build:

- risc-v architecture using running kernel in S-mode, main clock running @ 75MHz, SD peripheral @ 25MHz
- Toolchain: riscv64-unknown-elf-gcc-8.3.0-2019.08.0-x86_64-linux-ubuntu14
- Nuttx GIT version: https://github.com/apache/nuttx/pull/8922 with local changes in littlefs to use kmm_malloc/free due to using DMA and S-mode kernel
- Built with commands:
```
$ make distclean -j16
$ ./tools/configure.sh rv-virt:knsh64
$ make -j16
$ make export -j16
$ make import -j16
```
- Executed with command:
` $ qemu-system-riscv64 -semihosting -M virt,aclint=on -cpu rv64 -smp 8 -bios none -kernel nuttx -nographic -serial mon:stdio`

Our architecture and hardware design requires the product to boot from eMMC and have a fast boot time for a firmware image of > 2MiB. At 50KiB/s the image load itself would consume >40 seconds.

NOTE: Interestingly littlefs performs considerably better than vFAT on SD/eMMC, which on the same setup has read/writes speeds of ~12KiB/s. Littlefs however on a RAM disk gives speeds of 560KiB/s for write which is considerably slower than vFAT, which measures ~3225KiB/s.

Does anyone have a rough feel on what performance we could expect when using littefs on SD/eMMC in Nuttx for a configuration like described above and are there ways to improve this?