Modern x86 (and IIRC SPARC) processors have hw accelerated CRC32C. "Luckily", the exact same polynomial is used in the btrfs and ext4 crc checksum, among others, allowing those to benefit from the hw acceleration.
Via's cpus have supported hardware SHA-1/SHA-256 for years. Many ARM cpus also support it in hw.
Does it checksum metadata only? Is there checksum support for file's data?
Also, CRC32 seems a little bit weak.
As far as i know, CRC32 is used by many hardware internally for error correction.
When data corrupted and undetected in hardware level (which means the hardware CRC32 test probably passed without error), i doubt how reliable that ext4's CRC32 can detect corruption.
At least an alternative implementation of MD5 should be provided, the overhead should be acceptable if those crypto library are written in SIMD instruction (MMX, SSE)