Speed prepot > (from this moment continue only when your long/strong *procs* hit if they didn't already) Hypersteed Accelerators > Speed potion > next Accelerators in 33 seconds ( > on cooldown)
On end-game gear you'll already have 253 haste from items so it's a nobrainer

"Does the answer change if you use mostly strength or arp gems?"
There is no such thing as mostly strength or arp gems. The only time you will have strength gems is while you're gearing yourself e.g. below 6k gearscore. That's not unholy - your AoE sux compared to theirs and there is no point going strength over arp for single target..as dual-wield FROST. At all.
Ok, thank you for the answer on the pots.
I asked about strength gems because I am still gearing my frost dk (5,8k actually, with a mediocre off hand from ICC 10N) and I have no arp trinket, so I am still strength gemmed. As soon as I will obtain the STS trinket I will switch to full arp.