That depends a lot on what you want to use it for. A Z80 can stream data to/from RAM at several hundred kilobytes a second, do binary search at tens of kilobytes per second, or calculate hundreds of floating point values a second with a softfloat library.
For basic word processing on a terminal, that is fast enough to support potentially dozens of simultaneous users. There were a number of software packages in the 1980s that would run a virtual machine interpreter, providing a multitasking multiuser environment with integrated high level languages, databases, office applications, etc. on 8-bit microprocessor hardware. Also plenty fast for a print server or similar, at that time. But, on the other hand, the 64 KB address space isn't really enough to fit even a basic optimizing compiler.
For basic word processing on a terminal, that is fast enough to support potentially dozens of simultaneous users. There were a number of software packages in the 1980s that would run a virtual machine interpreter, providing a multitasking multiuser environment with integrated high level languages, databases, office applications, etc. on 8-bit microprocessor hardware. Also plenty fast for a print server or similar, at that time. But, on the other hand, the 64 KB address space isn't really enough to fit even a basic optimizing compiler.