Or quadruple boot, is it slower than a dual boot?
For example Windows 10, Ubuntu, Arch Linux.
I did a quadruple boot once and I felt that it was so slow all the boot process, but I don't know if it may be a bad installation.
My five cents into this downvoted question:
Dual, triple or XYZ-ipple boot should not affect properly-configured system.
It will take few milliseconds for bootloader to check for drives availability, nothing more than that. After OS boot process was initiated by bootloader, one can forget about other boot options
Still, from my personal experience, with dozens of HDD/SSD's and partitions of various structure and filesystems (e.g. hardware RAID, Software RAID, NTFS, ext3/4, XFS, FAT32 et cetera - I do have pretty weird seup sometimes), it can take much more time (very much time) for bootloader (GRUB) during partition scan (during OS install or configuration refresh). That's the only significant issue with it I can think about