My desktop CNC milling machine came with under 4krpm top speed from a DC motor which was very disappointing in use, I converted it to 3 phase which gives me up to 7krpm, for smaller work I built a simple speed increaser which gives me up to 30krpm but ideally buy something which covers as large a range of work as possible both speeds and envelope because jobs will come along which you could do "if only" ;-)