Author of ripgrep here.

It really just depends. The way I like to characterize `git grep` (at present) is that it has sharp performance cliffs. ripgrep has them too, to be sure, but I think it has fewer of them.

If you're just searching for a simple literal, `git grep` is decently fast:

    $ git remote -v
    origin  git@github.com:torvalds/linux (fetch)
    origin  git@github.com:torvalds/linux (push)

    $ git rev-parse HEAD
    f1fcbaa18b28dec10281551dfe6ed3a3ed80e3d6

    $ time LC_ALL=en_US.UTF-8 git grep -c -E 'PM_RESUME'
    Documentation/dev-tools/sparse.rst:3
    Documentation/translations/zh_CN/dev-tools/sparse.rst:3
    Documentation/translations/zh_TW/sparse.txt:3
    arch/arm/mach-omap2/omap-secure.h:1
    arch/arm/mach-omap2/pm33xx-core.c:1
    arch/x86/kernel/apm_32.c:1
    drivers/input/mouse/cyapa.h:1
    drivers/mtd/maps/pcmciamtd.c:1
    drivers/net/wireless/intersil/hostap/hostap_cs.c:1
    drivers/net/wwan/t7xx/t7xx_pci.c:15
    drivers/net/wwan/t7xx/t7xx_reg.h:7
    drivers/usb/mtu3/mtu3_hw_regs.h:1
    include/uapi/linux/apm_bios.h:1

    real    0.215
    user    0.421
    sys     1.226
    maxmem  161 MB
    faults  0

    $ time rg -c 'PM_RESUME'
    drivers/mtd/maps/pcmciamtd.c:1
    drivers/net/wwan/t7xx/t7xx_reg.h:7
    drivers/net/wwan/t7xx/t7xx_pci.c:15
    drivers/net/wireless/intersil/hostap/hostap_cs.c:1
    drivers/usb/mtu3/mtu3_hw_regs.h:1
    drivers/input/mouse/cyapa.h:1
    arch/x86/kernel/apm_32.c:1
    Documentation/translations/zh_CN/dev-tools/sparse.rst:3
    Documentation/translations/zh_TW/sparse.txt:3
    Documentation/dev-tools/sparse.rst:3
    arch/arm/mach-omap2/pm33xx-core.c:1
    arch/arm/mach-omap2/omap-secure.h:1
    include/uapi/linux/apm_bios.h:1

    real    0.078
    user    0.259
    sys     0.577
    maxmem  15 MB
    faults  0

But if you switch it up and start adding regex things to your pattern, there can be substantial slowdowns:

    $ time LC_ALL=C git grep -c -E '\w{5,}\s+PM_RESUME'
    Documentation/dev-tools/sparse.rst:1
    Documentation/translations/zh_CN/dev-tools/sparse.rst:1
    Documentation/translations/zh_TW/sparse.txt:1

    real    5.704
    user    55.671
    sys     0.585
    maxmem  207 MB
    faults  0

    $ time LC_ALL=en_US.UTF-8 git grep -c -E '\w{5,}\s+PM_RESUME'
    Documentation/dev-tools/sparse.rst:1
    Documentation/translations/zh_CN/dev-tools/sparse.rst:1
    Documentation/translations/zh_TW/sparse.txt:1

    real    24.529
    user    4:34.42
    sys     0.753
    maxmem  211 MB
    faults  0

    $ time LC_ALL=en_US.UTF-8 git grep -c -P '\w{5,}\s+PM_RESUME'
    Documentation/dev-tools/sparse.rst:1
    Documentation/translations/zh_CN/dev-tools/sparse.rst:1
    Documentation/translations/zh_TW/sparse.txt:1

    real    1.372
    user    16.980
    sys     0.647
    maxmem  211 MB
    faults  1

    $ time rg -c '\w{5,}\s+PM_RESUME'
    Documentation/translations/zh_CN/dev-tools/sparse.rst:1
    Documentation/dev-tools/sparse.rst:1
    Documentation/translations/zh_TW/sparse.txt:1

    real    0.082
    user    0.226
    sys     0.612
    maxmem  18 MB
    faults  0

In the above cases, ripgrep has Unicode enabled. (It's enabled by default irrespective of locale settings. ripgrep doesn't interact with POSIX locales at all.)