A good criteria to judge if a tool will be fast is often the package size. Tools in or under 10s of MBs usually do one thing and do it really well.
Tools listed here have fast/instant startup and/or handles large files without slowing everything down.
- Notepad++ https://notepad-plus-plus.org/
- Notepad2 Notepad3 (not sure if these are actually fast)
- SumatraPDF https://www.sumatrapdfreader.org/free-pdf-reader
- IrfanView https://www.irfanview.com/
- XnView https://www.xnview.com/en/
- JPEGView https://github.com/sylikc/jpegview
- Everything https://www.voidtools.com/
- Ripgrep https://github.com/BurntSushi/ripgrep
- Nirsoft tools https://www.nirsoft.net/
- Sysinternals tools https://learn.microsoft.com/en-us/sysinternals/
Uusually all small and fast softwares here https://www.portablefreeware.com/