Music performance setup - with HDD + SSD + more SSDs?

Question: I'm aware that the OS runs much faster on an SSD than on a HDD.

I'm still confused though. I want to do live DJing with my computer.

Essentially l need to do 3 things very fast:

Fast 1: Run DJ software
Fast 2: Load songs and samples (to trigger alongside the songs)
Fast 3: Record everything on a multitrack music program

As l see it, running the OS (booting up OS, opening browser, opening calculator, changing font size etc. etc.) need not be so fast. Therefore am l OK to relegate it as:

Slow 1: Running OS

... or is it that the OS's activity is intertwined with Fast 1, 2 & 3? And therefore the OS would also need to be fast?

My current plan:

HDD: For running the OS
SSD: Install DJ program here. Also install multitrack program here. Also store songs and samples.

This also raises another question: surely, recording into a multitrack program, whilst simultaneously also playing tunes via a DJ program, all on the same SSD drive, with the songs and samples also being loaded on that SSD drive, would slow the SSD down?

Therefore should I have at least 1 more SSD for the multitrack recording? And perhaps 1 more for the tunes and samples?

Please help, as I'm about to add a million SSDs to my shopping and this could be costly. I hope I'm missing something here, and that all I will need is 1 HDD and 1 SSD.

Oh and there's the original question as well: could the OS go on the HDD rather than the SSD?

Assumptions: the computer is equipped (speed, memory, drive size) for the task.

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