Some things like this do exist. Some programs or even entire customized distros will LD_PRELOAD jemalloc for size or hardened malloc for security (I'm sure there's a performance-optimized version out there I don't know about). There's nothing at all stopping developers from providing optimized subsets of libc intended for more specific use cases than the fully-general system-wide libc.

Note that when you're comparing Centos 6 to Debian 10, you're comparing a system released in 2011 that went end of life last November to a system released in 2019. I don't think it's that surprising that 8 years of development brought a 500% performance increase. Presumably, Centos 8 (released 17 days after Debian 10) would have had the newer glibc.