I don't believe there are DDoS attacks. The 'DDoS' attacks, as Serpens66 pointed out, always coincide with strong price movements. Kraken's customers can't take advantage of obvious trading and arbitrage opportunities. Usually this is unmistakable sign of insider manipulating profit taking and margin liquidation price levels.
No way that Kraken would risk doing just that. (IMO)
Plenty of other possible bottlenecks "could" play a part (just brainstorming here) :
Just to name a few:
- server/hardware configuration/setup (tweaking/upgrade needed?)
- database processes handling (tweaking needed?)
- general coding (rewriting parts of code to use resources more effective - that takes long but often makes the difference)
- Cloudflaire protection filter tuning (balancing between fastest possible access of clean traffic while malicious traffic gets blocked)
etc etc.
(edited for wording)